But still the dog, a Maltese terrier, kept talking, and over the ensuing decade O'Hagan followed its little paw prints across Hollywood and beyond in between writing a trilogy of internationally acclaimed novels that redefined a kind of Scottish legacy of the mind and heart.

The trilogy began with Our Fathers and ended with his Booker long-listed 2006 novel, Be Near Me, which charts a priest's fall from grace after an ill-judged relationship with a 15-year-old boy and sealed O'Hagan's reputation as one of the most distinctive novelists writing in the English language today. 'Sometimes you've just got to show a little bravery in relation to what comes up in your head, and actually pursue it,' O'Hagain says. 'And once I started following this dog, literally, though Hollywood, and through many, many interviews with people who had been in the movie business or in psychoanalysis or in the Actors Studio, during those years, I knew that the book absolutely belonged to me.'

Indeed it is hard to think of a writer who could pull off the same undertaking with such verve and originality. The voice of Maf, who narrates The Life and Opinions of Maf the Dog, with a warm, wry humanity and philosophical and literary insights, is a triumph, as indeed is the voice of Frank Sinatra, who gave the dog to Monroe in November 1960, two years before she died. Monroe named it Mafia Honey, Maf for short, in honour of Sinatra's mobster connections, and Maf's witty, wise recall of this event, together with his 'terrifically intact' Scottish pedigree and encounters with famous humans during his years with Monroe make for sparkling comic reading.

With a stellar cast that includes not just Sinatra, but Peter Lawford, Natalie Wood, Lee Strasberg, Shelley Winters and Carson McCullers and Kennedy, to name but a few, Maf sets the tone for his narrative by declaring early on that 'people have no head for miracles. They are pressed into shape by the force of reality, a curse if you ask me. We have none of that fatal human weakness for making large distinctions between what is real and what is imagined.'

Little wonder that movie rights to the novel have already sold to Stone Village Productions, the Hollywood producers that brought The Human Stain and Love in the Time of Cholera to the screen. But this sparkling picaresque comedy of ideas is not, as some critics have claimed, an homage to Monroe, but a superbly nuanced portrait of the American century, of an age that was, says Maf, 'all about hope'. It's no mere whim that has Maf bear witness to Monroe's 'rituals of becoming' along with the birth of America's new liberalism, civil rights movement and the space race.

'It is a kind of democratic staple, but the nature of hope has changed over the decades,' says O'Hagan. 'Now there is a sense of 21st-century cynicism about the possibility for change, so I wanted this book to reach back into that previous period of hopefulness. To actually look at humanness in the only way that seemed completely right and natural to me, through a creature's eyes, participating in that literary tradition where animals can see things about humans they can't really see themselves.'

He doesn't see the novel as the radical departure for him that reviewers have claimed, but a continuation of his preoccupation with the nature of transformation, of re-invention, the central theme of all his books. But even more than that, says O'Hagan, it echoes the central theme of his own life. And while The Life and Opinions of Maf the Dog is the first of his novels set in a gritty, grainy Scottish town, it carries into prose his experience of what it was like, growing up on a council estate in Scotland's Clydeside. 'Of course the circumstances clearly weren't those of a small Maltese terrier,' he says, 'but the dog is questing for a purpose and for a style of expression and for a way of capturing life as it is happening, and that has been the story of my life.'

O'Hagan has often spoken of growing up in a house where the only book was the telephone directory, but when his father brought home a tattered copy of a biography called Norma Jean: The Life of Marilyn Monroe by Fred Lawrence Guiles, 'it felt like a revelation', recalls O'Hagan who was 11 at the time. 'Suddenly there was a story of this poor girl who had tried to imagine her way into another sort of life, and it just chimed with me. It just felt like a very magical, mythic story. We forget that for children born in the 60s, as I was, that period was a kind of cultural revolution. Myths could be found in the news, they could be found in everyday life.

'Celebrity culture is about the way these archetypes come to be present in your life, and the story of this girl seemed to me as classical as any story of the mythic sort. Over the years it seemed to define the relationship between reality and imagination for me.'

Famous for the power, humanity and moral force of his sentences on the page, O'Hagan is just as dazzlingly eloquent off it. He speaks of his own childhood and of the way popular culture impinged on his life - and continues to impinge on all our lives - with infectious ebullience. It was Monroe, he maintains, 'who opened the door' for him, however inadvertently, to other areas of 20th-century culture, to painters, musicians and writers. But before Monroe, he longed to be a ballet dancer. 'My father nearly had a heart attack when, in the middle of this working-class housing estate outside Glasgow, the youngest of the family decides to venture forth in his ballet pumps to the Jacqueline Thompson School of Dance and throw himself from one end of the hall to the other in a dramatic way. I wanted to test the boundaries, and that dance class was the start of all that. Then came Monroe and the whole thing started to connect up.'

In some ways, argues O'Hagan, 'Monroe is a kind of patron saint of becoming. She's an icon of invention, she stimulates the need for invention in other people. We're not just solid clay that's already formed and, as it were, baked in a kiln never to change. We're constantly changing and the nature of that becoming is the story of our lives.

'I think so many problems we face in the world are to do with people expressing a free-falling anxiety about belonging in this tribe and not that tribe, of people being too certain of who they are as opposed to who other people are, and perhaps one of Maf's small contributions is to argue that who we are is much less important than who we could be.'