Everyone loves an outsider, the dark horse who comes up out of nowhere to win the big race. And who, until a year ago, had heard of Joan Brady, born American but now naturalised British? Not many, and small wonder. For 25 years she had lived quietly in Totnes, Devon, with her adored, much-older husband and their one son, Alexander. She published the occasional book and so did her husband but mostly they concentrated on one another. Last January Andre Deutsch published Theory of War - a novel her US agent had rejected. It received rapturous reviews and won the 1993 Whitbread Novel Award.

Joan Brady's life story is nearly as astonishing as that of her grandfather, the subject of the novel.

'When I was three I was in love with my cousin, Garland Ethel, because of his lovely name, with Robert Oppenheimer, until I decided I didn't like the way he held his head, and with a friend of my parents, Dexter Masters. I always knew Dexter was the one I wanted to marry. There were verities: mother, father, God and Dexter. He was an eternal verity.

'He was funny and amusing, and I was enchanted with him. I didn't know until much later that he was my mother's lover, or that after my father's death, when I was 17, she had picked Dexter for her second husband. I knew they were friends, of course, but that was all.

'I liked the way he looked; he was a very beautiful man.' A photograph shows a square, confident face with steady eyes and springy hair; the kind of man who would attract plenty of women. Joan never had any other serious relationship. At 17 she was accepted at George Balanchine's American Ballet School, enabling her to follow Dexter Masters to New York. 'I had decided that Dexter was the guy I wanted and I was going to get him, no matter what.' On arriving she stayed at the YWCA for Young Ladies in the Arts on 77th Street, but a few months after the death of Dexter's wife she moved in with him.

'I think he was more flattered than anything. I thought he was absolutely magnificent and adored everything he was, and what could be more seductive for a man? Plus, as somebody said, there's nothing as beautiful as young flesh, and I was well endowed with it.'

Living together was much more shocking in the late Fifties, though even today many people would balk at an 18- year-old living with a newly widowed man of 50. 'I was the one who proposed. He said, 'It won't last', and I said, 'OK, let's try it for a year.' ' When they married, her husband had to give up smoking because no one would sell Joan cigarettes. 'I looked under 14.'

She gave up dancing, although by then she was with the New York City Ballet, and took a degree in philosophy at Columbia. While she was pregnant, her mother died. 'I found it difficult. People assume you have orderly, consistent feelings but however much you fight with somebody there is that conversation. All the things you know nothing about, all gone; all, now, completely beyond you.'

In 1965, after their son's birth, the couple settled in England, educating Alexander at Bedales. Four years ago, after nearly 30 years of absorbingly happy marriage, Dexter died. Joan Brady resumed work on Theory of War.

It is entered in the novel category for the Whitbread Book of the Year award, whose winner is announced tomorrow. Like Alex Haley's Roots, it shows family history twining itself around the lives of several generations, distorting people as yet unborn, sometimes irreparably.

Brady's Irish great-grandfather was pressed into the Navy at the age of 14 - as near to slavery as makes no odds - and her grandfather, sold for dollars 15 at the age of four, was also to all intents and purposes a slave. Both were white. Losing all contact with his family, her grandfather (fictionalised as Johnny Carrick) worked for a poor mid-western farmer. Living in a mud hut, unpaid and cruelly treated, he escaped 12 years later to plot and execute a fearful revenge.

Her grandfather's anger and bitterness and frustration at having been deprived of parents, childhood and all capacity for happiness warped the lives of his seven children and still resonates through Joan. Perhaps now that she has written it down the fury will have been exorcised.

Joan Brady lives in a former wool merchant's house. Light pours into her long drawing room from the garden beyond. The walls are hung with eclectic paintings - many by Dexter's first wife, a New Yorker artist - and hand-made rugs lie over wooden or stone-flagged floors. Joan herself, dressed in drifting black from head to foot, is slim and lithe: she admits that hers is the ideal dancer's physique. Grey hair folds into a neat duck's tail at the back of her narrow, finely boned head. She talks with a disarming mixture of openness and intensity. She lacks the awkwardness sometimes found in people who live alone, and is thrilled by the success of her book.

How much of her grandfather's story, as told in the novel, is literally true?

'The outline is true, and the details which depend on historical research are accurate. The sale, the conditions under which he was treated, his first attempt at murder, his running away to work on the railroad, and the seven children born to him later: all that is true. But his character is invented - he died before I was born - although I knew about him from the time I was tiny. It was only when I was over 40 that it occurred to me that here was this story nobody had used.'

The legacy passed down to his seven children was a heavy one. 'Four committed suicide, of whom one was my father; one was mentally handicapped, and one drank himself to death. Only the oldest survived more or less intact. My grandfather did not understand anything about dealing with a family or what a child needs. When they were little he worked them hard in the fields. He also built them swings, and because they still had energy left to play he figured he couldn't be working them too hard. He'd never played himself; he just didn't know what it was for. He was a highly damaged person, though also highly intelligent.'

Her grandfather Nathaniel Brady's fictionalised search for his father is one theme of the book: everyone's need to find their roots and biological origins.

'He discovered a Union soldier in the Civil War called Daniel Brady, who did contract a sale, and that there had been other children. He managed to find them - no one knows how - and took care of them all. None of the others had been 'bound out' or sold; apparently Daniel Brady believed my grandfather had been a bastard, which is not unlikely.'

Joan Brady sits serenely in her quiet book-lined house in a medieval street several thousand miles away from San Francisco, where she was born. On one wall hangs a Cartier-Bresson photograph from the Fifties of George Balanchine taking a ballet class. The long-legged sylph in the foreground is herself. Now she is taking a physics degree through the Open University, and was thrilled to have been marked alpha for last term's work. Her son lives in Cambridge with his girlfriend. Has her grandfather's legacy finally been outlived?

'I think I was lucky. I'm more like him than most of the others . . . really quite tough. My sister was much more troubled. She had to have many years of therapy, and her therapist said she had all the hallmarks of an alcoholic's daughter. Alcohol had no part in it. Yet I think the reason I was so happy to get out of the family and into ballet was that here was order: something you could understand and master by physical discipline.'

Her writing style is a tense, precise distillation all the more effective for being understated. Does she write slowly?

'Theory of War took 10 years. I could not write the way I do until after Dexter had died. He liked me to write in a 'feminine' way, and many of what I now regard as the best passages in the book are ones he took out originally. He wanted me to seem gentle and sweet, not tough or masculine.' She smiles. 'Also, I stole a lot. I used to steal as a dancer, too. I would copy someone else's movement and make it my own.' She elongates her arm and with an exquisite curl of the wrist and inclination of the fingers makes a balletic gesture. 'You can do it in ballet and in writing.'

She claims that the best sentence in the book was suggested by her husband.

She turns to page 15. 'I can't tell you how I agonised over this. It's crucial, and I must have rewritten it a hundred times. In the end Dexter said, 'Why not just say 'I want to buy a boy.'? ' And there it is. That's it. The whole story of the book in those six little words.'

'Theory of War', Andre Deutsch, pounds 14.99.

(Photograph omitted)