“I have been interviewed by the Guardian before,” William Trevor declares triumphantly when, finally, we meet. “It was way back in 1964, so maybe it doesn’t count.” For such a prolific and internationally celebrated writer, Trevor’s reluctance to submit to the demands of today’s publicity machine almost qualifies him for reclusive author status. It’s no exaggeration to say this interview has taken several years - and a small cache of postcards in the author’s tiny handwriting - to set up. “But I do always say no,” he concedes. “It kills you in the end, anything you are doing that isn’t just writing. It’s no joke.”

The last significant interview was 20 years ago for The Paris Review. His then interrogator observed that his ability to inhabit so completely the lives and thoughts of such a wide range of characters was “disturbing”, “somewhat diabolical” even. Trevor’s extraordinary empathy is undoubtedly one of the defining qualities of his work, but it is hard to imagine a less diabolical figure than the twinkly 81-year-old, in schoolmasterly green cords and cardigan, sitting tidily on the sofa. Quite the opposite: there’s something almost monk-like about his aura of good-natured serenity and wisdom. This comes perhaps, in part, from what - on the couple of occasions he has been pressed to talk - Trevor claim s as a complete lack of interest in himself. “It’s true,” he says, when challenged - solipsism, after all, is often assumed to be a necessary part of the writer’s toolkit. Not for Trevor. “Other people interest me far more. Other people fascinate me.”

Despite his reticence, he has a natural storyteller’s ease recalling his past, sometimes almost as if he were remembering somebody else’s life. As he cautions: “Everything I say is sort of a guess. There is a tendency, I think, especially if you’ve been around for a long time, to almost exaggerate, you add a little bit yourself to almost any chunk of life you care to mention, and I find myself doing that. I know I do.”

Trevor’s reputation as one of the world’s finest short story writers has grown quietly but steadily over the half century he has been writing: as long ago as 1975, Graham Greene described Angels at the Ritz as “one of the best collections, if not the best since James Joyce’s Dubliners “. His insistence on the inexplicable, almost mystical, nature of the creative process, describing himself as “an absolutely instinctive” writer, adds to the sage-like impression. “To me, writing is entirely mysterious. If I didn’t believe it was a mystery, the whole thing wouldn’t be worthwhile. I don’t know not just how something is going to end, but what the next couple of lines are going to be.” He is almost superstitiously reluctant to analyse how he does it: “If a great cricketer like Viv Richards, or a great tennis player like Federer, studied how he did it, then he would lose it. Something happened to Federer in the course of the last 12 months, he lost the whole game, then he came back. No critic has ever suggested that he knows the reason for it.”

One reviewer described The Story of Lucy Gault as “quite possibly the saddest story you have ever heard”. And a profound melancholy pervades all Trevor’s fictional landscapes as softly and persistently as Irish rain. His tales of quiet calamities and defeat, disappointment and guilt, are saturated in loneliness and secret sorrow. Why is his fiction - unexpectedly dark at times - nearly always so heartbreaking?

“They’re not all sad, not all sad from start to finish. My new novel is not particularly sad. But the answer to the question is I just don’t know. It doesn’t come from an inner pain of my own.” Love and Summer, Trevor’s 14th novel, as brief and captivating as its simple title suggests, is on the long list for this year’s Booker prize which, despite having been shortlisted several times - “along with Beryl [Bainbridge]; she’s one or two ahead of me” - he’s never won.

While not, perhaps, as devastating as Lucy Gault or Felicia’s Journey, Love and Summer contains all the vintage Trevor hallmarks: past shame; secrets; sacrifice; and, finally, redemption - or consolation, at least. “If you take away the sadness from life itself,” he muses, “then you are taking away a big and a good thing, because to be sad is rather like to be guilty. They both have a very bad press, but in point of fact, guilt is not as terrible a position as it is made out to be. People should feel guilty sometimes. I’ve written a lot about guilt. I think that it can be something that really renews people.”

Hoping for a happy ending to her story, poor Lucy Gault reflects: “novels were a reflection of reality, of all the world’s desperation and of its happiness, as much of one as of the other. Why should mistakes and foolishness - in reality, too - not be put right while they still might be?” But Trevor seems to take an almost Hardyesque delight in thwarting his characters’ - and readers’ - desires; in denying them the chance of ever fully transcending their mistakes and foolishness. As New York Times critic Michiko Kakutani observes: “The line of demarcation in a character’s life has less to do with the loss of love than with the loss of innocence - something happens to fundamentally change how an individual sees himself or his family or a friend; and in the wake of that revelation, his entire relationship to the world is altered.”

“My stories have an awful lot to do with feelings, they aren’t to do with other realities. All kinds of feelings seem to me to be worth going on about in print, exploring and wondering about and being curious about,” Trevor says. “I write out of curiosity more than anything else. That’s why I write about women, because I’m not a woman and I don’t know what it’s like. The excitement of it is to know more about something that I’m not and can’t be.”

Trevor is still regarded, both by readers and himself, as “an Irish writer” even though he left his native country in the 1950s. Today he lives with his wife of more than 60 years, Jane, in a Victorian farmhouse opposite an old mill they bought many years ago in some of Devon’s loveliest countryside, spending several months of each year in either Italy or Switzerland. While he doesn’t quite start at the punishing time of 4am, as he used to, he is still at his desk every morning between six and half past. For those couple of hours before breakfast, he says, he has the same energy and sharpness as when he was 16 or 17. He writes on blue paper, “an affectation”, on one of four Olympia typewriters, “a German one, the best machine in the world”: Jane is in sole command of the computer. A great deal of his work is rewriting, a process he compares to cutting and editing a film - “what you leave out is the most important” - often revisiting stories many months later. He works until coffee at 11, when “a great deal of conversation takes place, which should be my good writing time, but I can never resist talking to Jane.” The rest of the day he works in his garden.

While a writer’s life seems idyllic, there are, perhaps, clues to the tragic sensibility of his fiction in his past. He was born Trevor Cox in County Cork in 1928 to “lace-curtain Protestant” parents who hated the sight of each other. His own, very happy, marriage (they have two sons, a barrister in London and a television presenter in Boston), he says, is the product of of two appalling marriages, his parents’ by far the worst. “It wasn’t so much that they quarrelled; I never heard my father shouting or anything like that. They just simply didn’t get on. There was no respect, nothing.”

There were three children in what he remembers as a very lonely household. His sister, now dead, really had no life at all, he reflects, as a result of being caught between warring parents. And it is only now, as old men, that he discusses this miserable childhood with his younger brother. “All the way on the bus home, he used to dread coming back from school: would they be friends or would they not be friends? Most of the time they were not friends, there was total silence in the house, nothing said at meals.”

His parents finally split up, but only after his father had retired and the children had left home. He still can’t understand “why these two very attractive people just couldn’t put the thing together. I’ve always thought,” he says, his writerly curiosity at work again, “that something actually happened, the way that quite often in a marriage or a relationship something happens, and nobody knows because it’s kept away from the rest of the world, because there’s shame or something. There’s a big question mark.”

On account of his father’s job in an Irish bank, theirs was a peripatetic upbringing where each promotion meant a move to another small town. As a result, he attended 11 schools in total, often with periods of no schooling at all. “We had a girl who came to teach us, she didn’t know very much, but she was very nice.” He was, he says, quoting Agatha Christie, terribly lucky to have had such a poor education: “you take life more easily as a child, you don’t care if you do well or badly.”

He was useless at everything, he says, except composition. Each week they were set an essay, and while everybody else was writing about shamrocks or St Patrick, he would take the title and write a story. “I had a reputation for being very clever, which wasn’t true at all. What you are is instinctive and imaginative. They aren’t at all the same as being clever.”

He read detective stories and thrillers in “enormous quantities”, and decided upon a career in journalism, “anything in order to be able to write”. His plan was to leave school at the first opportunity and go to England to work on a provincial newspaper. But his mother persuaded him that his “puny efforts” would count for nothing when all the English journalists returned from the war. “And of course she was right, that wouldn’t have worked at all.”

So he went, reluctantly, to university to study history, and promptly lost all interest in becoming a journalist, preferring to spend his time carving and modelling. It was at Trinity College Dublin that he met Jane, and on finishing his degree he was briefly a teacher. After his school went bankrupt, the young couple emigrated to England, and Trevor eventually set himself up as a sculptor in the west country, “rather like Jude the Obscure without the talent”. It became clear that there wasn’t much money to be made from sculpture. For 16 years he didn’t write at all, but he decided to try his hand at a novel, using an Olympia that they had received as a wedding present. It made only the £75 publishing fee, and he has since disowned it; there is no mention of it on his - now prodigious - backlist. It was only, ironically, when he took a full-time job at a London advertising agency that he really began writing, at the age of 32. “It was a horrible job,” he says, but “they didn’t bother” him too much so he hammered out his first stories when he should have been writing ads for fancy fabrics and wallpapers. In 1964, his official first novel, The Old Boys, was published and Trevor Cox became William Trevor.

This autumn a complete collection of his stories, spanning nearly 50 years, will be published by Penguin. He still describes himself as “a short-story writer who happens to write novels. Not the other way around.” A short story, he says, is “a glimpse” of someone’s life or someone’s relationship. “You can take a relationship and almost photograph it. And there it is. Often that relationship can get lost in the bigger shape of the novel. I like to isolate it and really look at the characters.”

He enjoys switching between the two - although he’s not sure he will be able to do it again. “There’s a great relief at the slowness of a novel. On the other hand, there’s an even greater relief, if you stop halfway through a novel, and write a couple of short stories, they are so short, crisp and different. I think my novels have an awful lot of the short story in them. But I don’t think short stories have much of a novel in them, except they’ve all got plots, they are all stories - none of your modern stuff, you know!”

He was thrilled that Alice Munro (the world’s other “greatest living short story writer”) won this year’s International Man Booker Prize. But he’s not convinced about the much-discussed short fiction renaissance. “Short stories have never been terribly popular in England. They work very easily in America and Russia, but in England they are never quite as recognised. It isn’t that they aren’t as good. Whereas in Ireland, maybe because we don’t have that great tradition of the Victorian novelists, short stories have grown as rapidly as in America.”

One review of The Old Boys noted “Mr Trevor’s witty, ungenerous and unkind morality”; 20 years later a review of his stories commented on his “gleeful misanthropy” - both lending some support to the contention of recent critics that his work has “mellowed” and grown more serious over the years. Trevor denies both charges. “We do get more serious as we get older, but I would be very sorry about the laughter. I think there are several moments of laughter in the new novel.” He confesses that he has absolutely no interest in his novels or stories once they are finished, avoiding rereading his work unless he absolutely has to. But no, he doesn’t think there’s been any mellowing.

For many years he has enjoyed the security of a New Yorker contract for his stories, an arrangement he worries might make him complacent. “You need the freedom, especially when you get to a certain age, to do things you haven’t done before, you just want to try it out and see what happens. That sort of faint excitement might go altogether if you aren’t careful.”

One of the most striking things about his most recent collection, Cheating at Canasta, is the contemporaneity of its themes. In his novels, as in Love and Summer, he often looks back to recent history, while in his stories he is unafraid to tackle such modern issues as gang crime or internet predators as well as the more familiar material of infidelity and isolation. One of the most affecting tales is called “Faith”, about a preacher whose lost belief is restored - although in a less conventional “religious” sense - by the very ordinariness of his sister’s death. The story seems the closest to an articulation of Trevor’s own spiritual beliefs as you will find in his work. “Faith I liked writing very much,” he agrees. “That is my personal view. The whole shebang - angels and all the rest of it - is wrong, but there is something there. In an almost sentimental way you can look at the flowers in your garden and say, how on earth did He do all this? The word ’faith’ is used, but it is really a feeling. What they say in church every Sunday can’t be proved, but the feeling that people have can, because they have it.”

Trevor has been praised for his compassionate portrayal of evil characters, but he is “also very fond of writing about goodness”. It requires far more subtlety, he says: evil is pretty straightforward stuff. “I would use anything in order to tell a story,” he says with an unmistakable icy glint in those kindly eyes, “anything, anything at all to make the story work.” And you know that about this, at least, he isn’t exaggerating.

“No more. That’s all.” On her feet again to pour her second drink, Mrs Lethwes firmly makes this resolution, speaking aloud since there is no one to be surprised by that. But a little later she finds herself rooting beneath underclothes in a bedroom drawer, and finding there another bottle of Gordon’s and pouring some and adding water from a bathroom tap ... Opaque, blue to match the bathroom paint, the container she drinks from now is a toothbrush beaker, and holds more than the sedate cocktail glass, three times as much almost. The taste is different, the plastic beaker feels different in her grasp ... warmer on her lips. (After Rain.)

Chosen at random, this extract from a short story is an evening glimpse of Mrs Lethwes on a day when, waking early, she watched her husband sleeping and wondered if the girl he loved instead of her was awake also, if she shared with her the “same pale shade of dawn”, if there was, for her too, the orange glow of a street lamp”.

On yet another nothing day Mrs Lethwes continues to be haunted by her husband’s girl, whom she doesn’t know. She imagines short dark hair and elfin features, a small, thin body, fragile fingers. Mrs Lethwes’s day is a confusion of dread, of fear, of hope that isn’t real. And none of it, Mrs Lethwes knows, will go away.

No author’s comment need be made. It’s there, in Mrs Lethwes, in every minute that so slowly passes. Instinct wrote the story of a day, and how it did it is mysterious, as it always is. A reticent author delights in that.