It is the biography that is breaking records and astonishing the publishing world. Soaring high above other bestsellers, Billy , the life story of the comedian Billy Connolly, was found under more Christmas trees than any other new book.

Sales of the hardback in Britain and Australia have now reached 840,000, and the level of its success has rewritten the rules of the industry.

The candid biography, written by Connolly's wife, Pamela Stephenson, was first published by HarperCollins last October and serialised in the Observer.

Already on its 15th print run, Billy comes out in America this summer, but the sales figures so far have made the commercial impact of recent autobiographies by Victoria Beckham and Robbie Williams, at about 200,000, look limp in comparison - despite their £800,000-plus advances. HarperCollins is negotiating for Stephenson to take up another writing project: a novel, a biography or a comic work.

Stephenson, who was born in New Zealand and grew up in Australia, made her name in Britain during the 1980s with her comic impersonations of Janet Street-Porter and others on the BBC's Not the Nine O'Clock News . Since moving to Los Angeles with Connolly, Stephenson has retrained as a psychotherapist and her book makes play of its 'objective' stance, almost clinically charting Connolly's recovery from alcoholism and his attempts to deal with an unhappy childhood.

Before the book, Connolly was the famous funnyman who loved to shock audiences with his direct and bawdy manner. The publication of Billy showed this was only half the story. He has joined the ranks of those celebrities who are pitied as well as admired. The extreme poverty of his Glasgow childhood, the violence meted out by his aunt, and his sexual abuse by his father are all detailed in Billy .

'Stephenson's book is probably a special case, but many current hits rely on this triumph over adversity,' said Simon Prosser, of Hamish Hamilton, who is soon to publish a book about the difficult life of the founder of the Big Issue , John Bird. 'You can see it when you look at the sales of Dave Pelzer's book, A Child Called "It ".' Pelzer's first book, which chronicles his own child abuse, sold more than 600,000 paperback copies last year, while The Lost Boy has reached 400,000, and A Man Named Dave , which came out in hardback last January has already sold 180,000 copies.

'These redemptive books are not necessarily being read for their writing skills,'said Prosser, 'but because there is a peculiar pleasure in reading about how people have survived the kind of poverty or pain we don't all go through.'