A century of love, literature and letters: Diana Athill is still a force to be reckoned with at 93

When I first read Diana Athill's memoir, Instead of a Letter, I thought it was a tragic portrait of a middle-aged spinster who had been forced to find solace in books. Athill had written the book when she was 43 and an editor at Andre Deutsch. I read it when I was 25 and a publicist who hoped to be an editor at Faber and Faber. When I read it again this year, it sent shivers down my spine.

The book starts with the death of her grandmother, aged 94. While she was dying, she "turned her beautiful,speckled eyes" towards her granddaughter and asked: "What have I lived for?" Athill gestured towards the large family that "could not have existed without her", told her about the world that she had created for them and that "something that her love had made would still be alive". But what, she asks in the opening chapter, of a woman "who had never had the chance, or had missed the chance, to create something like that?" That, she says, "is a question to whistle up an icy wind".

What follows is one of the most chilling books I've ever read. A confessional memoir before the genre existed (it was written in 1962, way, way before the flood of children called "It" or hearts being smashed into a million tiny pieces), it recounts, with such cool precision that you have to read it to believe it, a life of happy high spirits and great privilege, soured by romantic disappointment. Brought up, at least some of the time, in a large country house with a butler, footman, cook, kitchen-maid, scullery maid, housemaids, lady's maid, coachman, chauffeur and gardeners, Diana Athill had a charmed childhood, full of the accoutrements – horses, hunting, balls – of a landed gentry background that fostered an "indecent sense of superiority". When she was 15, a young man called Paul (in the book, Tony in real life) joined the family as a tutor to her brother. "Within two days," she says, "the lines of my life were laid down".

The teenage Diana, who had been keen to explore sex since discovering a copy of Wise Parenthood on her parents' bookshelves when she was 11, promptly fell in love. Her infatuation didn't stop her pursuing other romantic possibilities, not even after she and Paul were engaged and she had happily surrendered her virginity. But Paul was the man she loved. Paul was the man she was going to marry. Paul was the man who would be the father of her children. When war struck and Paul went off to fight in the RAF, they wrote passionate letters about the life they'd lead together when it ended. Suddenly, the letters stopped. After two years of silence, Paul asked to be released from the engagement. He married someone else and died in the war.

By then, something had died in Athill, too. The pain of the silence had been like "a finger crushed in a door, or a tooth under a drill", but after that stiff little letter, her "soul shrivelled to the size of a pea". She continued to have affairs, "threadbare rags against a cold wind", and continued to work as an editor, with writers like V S Naipaul, Norman Mailer, Philip Roth and John Updike, but she discovered that "unhappiness was not a misfortune but a taint"; one that "substitutes for blood some thin, acrid fluid with a disagreeable smell". When she was 42, she sat down and wrote a short story and then eight more. The following year, she met a Jamaican playwright called Barry Reckford and started an affair that would continue, in various forms, for 40 years. "I have not," she says at the end of Instead of a Letter "been beautiful, or intelligent, or good, or brave, or energetic, and for many years I was not happy", but I have, she says (at 43!), "stopped feeling old".

"To die decently and acceptingly would," she concludes, returning to the theme that set her on this extraordinary confessional path, "be to prove the value of life and that, in spite of limitations and inadequacy, is what I have felt inclined – still feel inclined – and have a hunch that I will always feel inclined to do".

Instead of a Letter was written 50 years ago. Diana Athill is still going strong. Just how strong is evident as soon as she opens the door of her little room at the care home in Highgate, north London, where she now lives. In the garden, I've met a number of old women struggling with zimmer frames; each fighting a battle with frailty that's taking its toll. Athill, however, has the bearing of a queen. Her shoulders have the set of someone schooled in deportment, her skin the peachy, porcelain glow that generations of good breeding have buffed to a sheen. In flared, tweed trousers, a long spotty blouse, a kind of batwing cardigan thing and two strings of pearls, she is not just elegant, but magnificent.

"Would you like some tea?" she says, in the voice of a queen, too. It's a voice one almost never hears these days, even from The Queen, whose own cut-crystal vowels have softened with the passing years, but the kind that has one saying "one" all the time and feeling that cups of tea made by nearly-93-year-old literary ladies should be greeted with a curtsy. Instead, in a piece of vulgar name-dropping that has me feeling like the under-footman's daughter, I tell her – as I whip out my tape-recorder and balance it on the arm of a splendid red armchair that could almost double as a throne – that the last interview I did was with Gordon Brown. "Has he settled down?" she asks in the tones one might use of a particularly tricky child. "He'll be quite happy now," she adds, though it sounds, in her Brief Encounter voice, more like "heppy". "He should," she says firmly, "never have been Prime Minister".

She has, as it happens, just met another man she thinks shouldn't be holding high public office. "I had a long, lovely talk for a BBC programme with Rowan Williams," she explains. "I came to the conclusion that this is a man who should never have become an archbishop. He's not a man who ought to be wrestling with all these idiots. He's much too nice and too sensible". An atheist since her teens, Athill met the Archbishop of Canterbury for the same reason she has done pretty much everything in her life: intellectual curiosity. Also, because she was asked (along with Katie Price, who apparently turned it down) to guest-edit the Today programme over Christmas. If the resulting interview is anything like the grilling P D James gave Mark Thompson at the same time last year, it will be unmissable. Unmissable, too, because while preparing for the interview, Athill discovered something that Williams also knew: that Mother Theresa lost her faith as she began her work – and never regained it. "I thought," says Athill – and if you read her books and acclimatise to a voice that never offers praise that isn't absolutely felt, you feel the full force of it – "that that made her absolutely heroic".

Was she, I wondered, nervous about doing Today? Athill smiles and her blue eyes glitter.

"Not really," she says. "When you're old," she adds, in those thrillingly imperious tones, "you don't really mind." So does she never care what people think? She sips her tea and places her cup carefully down. "I care a bit," she says, "but you don't care all that much. It's not the end of the world. If they think you're a fool, you're rather sorry, but so what?"

It's hard to describe the delicious sensation that's triggered by such comments in that voice, as heady as the hyacinths sitting in a pot near the laptop on her desk. It's the sensation you feel when reading her books, too, the bracing feeling of muffled layers being whipped away. Ever since writing the worse thing she could think to write about herself, about the shame, grief, humiliation and the feeling of failure that was "always present like a river bed", the fear of what other people thought of her melted away.

Happiness crept up on her, after 20 years of the lack of it – and then liberation and then, at the age of 91, a kind of celebrity, when her memoir about ageing, Somewhere Towards the End, became a bestseller. Athill is now a regular on the literary circuit and loves it.

"It's fun!" she says. "It's an extraordinary thing. It's getting a little bit boring now, because I'm beginning to get bored of hearing myself say more or less the same things. But I have two new books coming up next year and that will give me other things to talk about."

The books, Athill is quick to point out, are a kind of "cheating". One is a collection of the short stories she wrote when she was 42, which were published in America – but not here – and which, until she got a letter from an editor at Persephone asking to publish them, she'd literally forgotten. The other is a selection of letters written to an American poet called Edward Field. When Field said he thought the letters should be published, Athill's response was: "Nonsense!" When he sent them to her to look at, she started laughing. "I thought: 'Oh, my God!'" she shrieks. "I'd forgotten that – and those letters were really an extraordinary portrait of a friendship and my life and what was going on in the firm and I thought: 'Perhaps they're right'."

When she sent them to her editor at Granta, she got a phone call giving her "an enormous offer" and the promise that they would make it next year's bestseller for Christmas.

There's something about the blue eyes, and the precision, and the enunciation, and the merciless eye in the books (but much more merciless to herself than others), which triggers confession. Before I can stop myself – and as I discover when transcribing the tape and find my so-called questions practically longer than her answers – I find myself volunteering all kinds of embarrassing snippets about my life. I was, I tell her, dumped by my first love when I was 19. (But I don't say "dumped". Like a Victorian governess, I say "jilted".) For many years after, I kept my heart locked up. I, too, worked in publishing and then in other handmaidenly capacities to writers, not having the confidence, or nerve, to do much writing of my own.

My most significant relationships have, like hers, been with black men. I, too, thought I would marry and have children, but haven't.

And I, too, in my forties, have been thinking, with quite good reason, about death.

Athill gazes at me with an expression of polite interest that suggests that such confessions are daily accompaniments to the Earl Grey tea. Embarrassed, I gaze back. She looks, I find myself thinking, monumental. Beautiful, in fact. "How can you say," I blurt out, "that you were never a beauty? That," I say, sounding like a doughty duchess, "is codswallop!" Athill smiles kindly. "I've become much better looking," she says matter-of-factly. "If you look at photographs of me, I was pleasant-looking. I had an interesting face, but I was not in the least anything like a beauty." But she clearly had what she calls "it", the thing that kept men flocking? The magnificent head swivels a micro-millimetre to one side. "I was too easy-going sexually, for a long time," she says. "I used to travel with my very beautiful cousin, Barbara. It used to make me so cross. People who fell for her used to always say: 'Marry me'. Mine used to say 'Come on, you know you'd enjoy it'. No one," she adds crisply, "ever felt romantic about me."

Somehow, I doubt that. After Paul, she was, she says, "terribly in love" with a man she met during the war. "I was his wartime folly," she explains. "My mother found out that he was married and she said: 'You know Stephen's married, don't you?', and I said: 'Yes', but I didn't and went out of the room very quickly. I thought: 'This means I can't go on with this', and then: 'But I'm bloody well going to'". She fell in love once more, in her early thirties, with a man who wasn't in love with her and told her so. After that, she settled for companionship and sex.

When she met Barry Reckford, he was married. She didn't fall in love with him, nor he with her, but they had many happy years together; the happiest before he moved in. Their sex life lasted, she says, "about eight years," and then she, "began to have to fake it". "One of the great things about Barry," she says, "was that he was extremely unpossessive. He believed in people being themselves. He didn't feel you were necessarily going to be bound to a person just because you'd had sex with them."

She, clearly, felt the same. In Yesterday Morning, her captivating memoir of "a very English childhood", published in 2002, she says: "I believe that if infidelity does not cause heartbreak in a spouse or deprive children of a parent and if it cheers up the two rule breakers, thereby adding to the pleasure abroad in the world, it does no harm". In Somewhere Towards the End, in which she writes with great frankness about sex and about her last affair, with another Caribbean man, which started when she was in her late sixties and ended in her mid-seventies, she says: "Where spouses are concerned, it seems to me that kindness and consideration should be the key words – not loyalty – and sexual infidelity does not necessarily wipe them out."

It's probably just as well. When she discovered that Barry was having an affair with a pretty young blonde called Sally, she was first upset – and then invited her to move in. "Honestly," she says, "one of the best things about the relationship with Barry was that he brought me and Sally in touch. She is an invaluable, darling friend. She flew here from Kenya for four days when I was moving into this home, to help me buy this chair and that desk."

Barry himself ended up crippled by diabetes and a "terrible burden". In the autumn of her life, and long after the relationship had stopped being sexual, Athill found herself taking on the role of wife. She was, in the end, "rescued by his relatives". "Thank God!"

It's the "big, fat cheques" she gets for Somewhere Towards the End – a massive surprise after an adult life of genteel near-poverty – which pay for her own care at the home in Highgate, which is, she says, "wonderful. There's no domestic worry left in my life. The hot water ceased the other day, and I thought: 'Oh, gosh, plumber', but I just say: 'Bob' – we have a wonderful handyman called Bob – and he sorts it out. He is," she adds, "the only man in the place".

Does it feel strange for a woman who has been used to spending so much of her time with men to live in a world that's largely female? Athill nods. "It felt very strange at first," she says, "but I'm used to it now." The other residents are all, she says, "lively and intelligent" and have all "had interesting jobs". How do they feel about this celebrity in their midst, this star reporter from the frontline of old age? A tiny smile. "They're thrilled."

It's 12.30 and, in old people's land, time for lunch. Athill lets me help her adjust her cardigan as she readies herself for the walk downstairs and tells me firmly that the collar of her blouse should not be buried beneath it. As we walk down a corridor, she points out a little garden which she helps a friend plant and which, she says, looks "beautiful in summer". As we walk downstairs, she points out the pictures on the walls. And, as we walk into the dining room, she points out her table. Forty-odd white heads are hunched over their food. For the first time, it really strikes me that Diana Athill is old.

I resist the urge, while waiting in her room, to snoop, though I do notice that the pictures are rather gorgeous and that amongst the many treasures on her specially-built bookshelves, in addition to hardback editions of her own books, are handsome sets of Jane Austen and Proust. I think how strange it must be to have your world shrink like this to a tiny room, but then I remember that a home isn't a world and that Diana Athill's world seems much, much bigger than most. She is, after all, a woman who hasn't just published some of the finest novels of the 20th century and written some of the finest memoirs of it. She is also a woman who has lived through nearly a century's history and whose observations on it sometimes take your breath away.

When she's safely back from a lunch of salmon, followed by ice-cream with bananas and chocolate sauce – and ensconced on the red throne – I ask her when she started thinking of herself as a writer rather than an editor who wrote? Athill stares at the big painting over her bed and then back at me. "Well," she says, "when I retired, really." She had written two memoirs after Instead of a Letter, one called After a Funeral, about an Egyptian writer and lover who moved in with her and then killed himself in her flat, and another called Make Believe, about a friend and lover who was a follower of Malcolm X and who was later murdered. She wrote them both for therapeutic reasons and then "put them in drawers".

It was 23 years after Instead of a Letter that After a Funeral was finally published, in 1986. Make Believe was published in 1993. When Athill retired, at 75, after 50 years in publishing, everyone urged her to write a book about it. "The thought of wading through 50 years..." she says. "I just thought: 'I bloody well can't'." In the end, she sent an extract about working with Jean Rhys to Ian Jack at Granta – and a few other bits – and he said it was "wonderful". "I adored that!" she says. "And it went very well and I got good reviews and that got me going".

The resulting book, Stet, published in 2000, is a fascinating portrait of a now lost world and of what it takes to bring a work of literature to birth. It also includes gripping accounts of two encounters with evil. One involved Gitta Sereny and her work on Into That Darkness, based on conversations with the Kommandant of Treblinka, Franz Stangl. The other was a meeting Athill had, about a possible book, with Myra Hindley. "It was very interesting meeting her," she says, "because she's so intelligent." Hindley had, she says, just "got to the point of agreeing to go up to the moors with the police to try and find the grave of the one child who hadn't been found" . "I thought," she says, "'if she is forced to face what she actually did, she will go mad'. I also thought: 'I don't want to murder this woman and that's what I'd be doing'. And as for the other side of it, the idea that the more we know about evil, the better we'll be able to cope with it, it's nonsense. We know as much about evil as we can and it doesn't make any difference at all."

On evil, as on so many other things, I think Diana Athill is right. I think that more information, about more cruelty, and more injustice, doesn't make us kinder. I think that writing is about "getting things down as they really were". I think that honesty is like a bracing breeze. I think, as she says in Somewhere Towards the End, that "we on our short-lived planet are part of a universe simultaneously perfectly ordinary in that there it is and incalculably mysterious in that it is beyond our comprehension", and that this does not feel like "believing in nothing".

And I think, as she did, after the miscarriage in her forties that nearly killed her, that other losses recede when set against the sheer, brilliant, screaming, shrieking joy of being alive.

Diana Athill will be guest-editing R4'Today' on 27 December