On the flyleaf of John Burnside's memoir, A Lie About My Father, is a brief caveat: 'This book is best treated as a work of fiction. If he were here to discuss it, my father would agree, I'm sure, that it's as true to say that I never had a father as it is to say that he never had a son.'

'But even before you get to that page, there's the title,' says Burnside, sitting opposite me in an Edinburgh restaurant. Though he's quietly spoken, the words spill out very fast, as if his ideas are in danger of piling into one another. 'On one level or another, it's a lie, partly because it's a one-sided account; there's no way I could write about this man and be impartial. I wanted to say, read this in a certain atmosphere, think about it.'

But the writer makes a contract with the reader, I point out, as Burnside knows well; now 50, he has published five novels and nine collections of poetry. The appeal of autobiography is that the reader can feel they are getting the facts unadorned.

'But it never is,' Burnside says. 'If I tell you a story, you can choose to believe me or you can question it. But a child isn't sophisticated enough to do that, although I realised very early on how much my father lied because his stories were always inconsistent. When I started writing this book, I quickly realised that it was a story in which lies and fiction and facts were interwoven for different motives, partly through a lack of self-respect on my father's part. I'd be doing the same thing with the reader that my father did to me as a child, except that the reader is in a position to say, well, I'm not sure about that.'

Burnside's father was a man of uncertain identity and many masks. A hard-drinking, Catholic working man in west Fife, known as Tommy to his friends and George to his wife, his unpredictable moods and violent temper kept his family cowering in fear throughout John's childhood and adolescence. Often he would disappear for days at a time, drinking away his wages while Burnside's mother begged for food from neighbours to feed John and his sister.

But it was the stories that marked his childhood most. Not the kind of stories that a child desires to be told by his father, where invention is exciting and by mutual consent. Burnside's father was full of self-aggrandising lies and he soon learned nothing his father said could be relied on. Although his father died in 1988, aged 62, it was only five years ago that Burnside uncovered some truth about the man for whom he once waited in an alley with a knife, intent on murdering him, and of whom he writes: 'I had carried him with me, an ember of self-loathing in the quick of my mind, caustic and unquenchable.'

'I'd never thought about writing an autobiographical piece,' he says, 'but when my son was born, I went back to visit my aunt in Fife. When you have a child, you think about your personal history and what you offer them as a larger narrative and I realised I knew nothing about my father's circumstances other than what he'd told me.'

Burnside knew his father had been adopted as a baby. What he learned from his aunt was that his father had been a foundling, left on a Fife doorstep in 1926. 'It made me think differently about him because he'd had to suffer things that, had he told me about them, I might have been more forgiving of his behaviour. I started writing this partly because I needed to work out how I felt about my father now that I knew his history.'

The second half of the book charts the legacy of that childhood in Burnside's own life. 'Bruno Bettelheim says in The Uses of Enchantment that children have to have magic in their lives,' he says. 'If there's no magic for them as children, they'll often do something later to create that magic and that can be very costly.'

For Burnside, that magic came in the form of LSD. Constantly striving for more extreme states, acid gave way to other drugs, week-long binges, heavy drinking and, eventually, two spells in a psychiatric hospital. Looking at him now - a lecturer in creative writing at St Andrews, the father of two small boys aged five and one, there is no outward trace of what he calls his 'fall', though the lyrical evocation of his state of mind during those lost years leaves the reader holding their breath.

'Someone asked me recently if this book is about the formation of a poet,' he says. 'I didn't want it to be like that at all; I wanted it to be just another working-class guy who got messed up and found his way out of it.'

In a distinctly unromantic twist, it was not poetry that saved Burnside, but computer programming. After his second stay in hospital, he moved to Surrey, got a job in software and sought to become the epitome of middle-class convention, another extreme, in its own way, but one that obliged him to find an alternative means of expressing his wilder nature. Unknown to his IT colleagues, he began writing poems, sending them to magazines, and had his first collection published at 33.

His father died the year his first book was published. 'I wouldn't have wanted him to read anything I'd written,' he says, bluntly. 'He'd discouraged me in everything I wanted to do, because of the climate of fear he lived in. Trust nobody; don't aspire above your station; you're poor so you're there to be hit and if you get used to that, you'll be fine. Just drink some beer and beat your wife up now and then.' He smiles sadly. 'My father was this big, tough guy, almost heroic in proportion to me as a child. It was only later that I saw how fearful he was. I think I'm much less fearful about the things that matter.'