How to draw a linguistic portrait of the wildly experimental, politically fearless artistic polymath that is Alasdair Gray? In A Secretary's Biography, the winner of this year's Somerset Maugham award, Glass models his biography on Boswell's portrait of Johnson. Every chapter marks a decade of Gray's life, interspersed with banter about Glass's day-to-day dealings with the "fat, spectacled, balding, increasingly old Glasgow pedestrian" (as Gray likes to describe himself). The result is both an accessible introduction for the uninitiated and an exciting opening-up of new vistas for anyone ever shaken and stirred by Lanark, Poor Things or 1982 Janine. What emerges most clearly is the sheer scale of Gray's inventiveness, as A Secretary's Biography discusses his lesser known paintings, plays and poetry alongside the novels (and what Gray was also doing while he wrote them). As big-hearted and unstintingly critical as Gray's own work, it is a fittingly entertaining tribute to this prodigious and vastly influential man of images and letters.

Rodge Glass and I have something in common: we have both failed to buy Alasdair Gray a drink. When I made my offer a few years ago, it met with polite refusal - and a short anecdote delivered with gusto on the subject of rank privileges and drink-buying in the Royal Navy. ('We don't,' he concluded, 'stand on that sort of ceremony here!')

According to A Secretary's Biography, Glass had a rougher brush-off. When Gray walked into the Glasgow pub where Glass was tending bar, the star-struck fan, having mixed the requested drink, launched into a question about the supposed 'postmodernism' of Lanark, Gray's best novel: 'Alasdair looked at me blankly. I tried to rescue the situation, "If you tell me, I'll buy you that gin and tonic." "Well ... " he muttered, not pleased. "I don't know. But I can pay for my own alcohol." He sank the thing quickly and left.'

There have been worse introductions - Boswell's meeting with the Scot-baiting Johnson comes to mind - and Glass was undeterred, winning a place as Gray's student on a university creative writing course and later becoming his secretary, friend and employee: 'He was paying me twice what I was getting in the bookshop... besides, it was fun waiting for the next entertaining Grayism to relate to my friends.'

Even in A Secretary's Biography, Glass's writing sometimes has the tone of a preferred undergraduate gossiping about his eccentric tutor, and the parts where Gray appears naked, drunk, rambling, giggling or dozing could well be headed 'Entertaining Grayisms'. No man is just a valet to his hero.

As well as a biographical document, this is Glass's PhD thesis, yet its critical insights are oddly sophomoric. 'At first look, Lanark is confusing'; 'another stylistic feature is how Gray personifies Glasgow'; 'Gray sees history in the broadest sense.' The last statement may be true, but Gray's head for detail is equally capacious, and detail is what this biography lacks. Glass would have done well to take a hint from Boswell, his stated model, who defended the 'minuteness' with which he recorded Johnson's speech. We hear tell of the Dictaphone Glass bought ('on Gray's request'), but he doesn't seem to have turned it on very often. At one point, Glass refers in passing to a conversation about Gray's 'single meeting with the avant-garde writer BS Johnson': I would gladly trade two-thirds of A Secretary's Biography for a full account of their exchange.

Among other things, Glass has learned from his former tutor how to deflect criticism with a pre-emptive literary strike. Gray's own self-deprecatory gestures have ranged from the innovative and witty 'index of plagiarisms' in Lanark to the cringing self-abuse of his last novel, Old Men in Love. But Glass seems not to have noticed that ironic autocriticism works only when the writing is strong enough to shoulder the irony. When he writes that his biography will be untidy, 'like Gray's work, life and much of his speech', he forgets that Gray's best books, elaborate as they are, are tidy and carefully controlled - a fact he elsewhere acknowledges in relating the obsessive methods and high production values which have infuriated so many of Gray's collaborators.

In other ways, A Secretary's Biography is a useful document. Its attention to the dramatic works is new and important, and it contains unforgettable scenes, such as Gray, just home from a month-long hospital stay, 'sitting at his old artist's desk reciting parts of Shakespeare's Hamlet to himself... dipping his brushes happily into a Baxter's Country Garden Soup can half-full with muddy water'. Nonetheless, when it comes to the life - as Gray says to his Boswell: 'It's all in Lanark, anyway.' And so it is.

There is nothing that diminishes a writer quite so well as a biography, nothing as certain to turn our idolised Ms Triumph into a Little Miss Wretch, or the much admired Mr Wonderful into the absurd Boy Snivelly. Rodge Glass's biography of Alasdair Gray could easily have become a classic work of biographical belittlement. Gray is, or has been, as heavy-drinking, as arrogant, as generally self-centred, as bearded, and as appallingly badly behaved as the rest of us, yet Glass manages to portray him without condescension or condemnation. He also manages, on the whole, to avoid the opposite of bio-miniaturisation, which is gross bio-puff-and-plumping and personality gigantism. The book just about keeps Gray in colour and life-size perspective: what we see is neither twit nor saint, but man.

Arguably, Glass is Gray's perfect biographer, having been taught by him at Glasgow University and then employed as his secretary for a number of years. Thus he has had the dubious privilege of having "seen him first thing in the morning, last thing at night, sober, drunk, excited, depressed, asleep" and, alas, "naked (passing me in the hallway on the way to the bathroom)". Glass is not unaware that this sort of close contact could also mean that he was "the worst person in the world to write his biography". Not necessarily the worst, but clearly not impartial either. He states that he's determined to avoid writing what he calls "A Sycophant's Bible of Alasdair Gray", but the book is inevitably, and not at all unpleasantly, suffused with a general tone of slight pleasantness: not a sycophant's Bible, but a Good Friend's Gemara.

The inside-track doesn't excuse Glass from doing his homework. He dutifully chronicles Gray's rather idyllic 1930s and 40s childhood in the Glasgow suburb of Riddrie, speaks to family members and friends, and catalogues his unique achievements as an artist and writer. Glass turns up various little nuggets, such as the Christmas 1947 issue of Whitehill School Magazine, with an early, typical Gray short story, and quotes diary entries suggesting that the beginnings of Gray's masterpiece novel, Lanark (1981), lie as far back as when Gray was 17. Grayaholics - Grayites, Graysons? - will doubtless also enjoy Glass's faithful account of Gray's Glasgow School of Art days, his chronicle of Gray's drifting in and out of teaching as a means of making a living, and his amusing retelling of the long saga of The Book of Prefaces, Gray's masterpiece work of non-fiction, finally published in 2000 after many years in preparation.

The summaries and analyses of the novels and short stories are undoubtedly useful (in particular the glossing of characters in Lanark), and Glass's sentence-length summary of what critics regard as Gray's flaws as a writer perfectly neat and handy ("the quality of Alasdair's output is limited by his need to pre-empt criticism, and bring socialism and Scottish nationalism into everything"), but it's probably safe to assume that anyone reading the book will already have read, or will certainly want to read, Gray's books for themselves and come to their own conclusions. What one really wants from a biographer, after all, is biography - a little taste of the juice in the fruit, or the gravy to go with the pie.

Glass does not stint on the juice or the gravy. He rightly probes Gray about his relationships with and portrayals of women, and returns again and again to discuss the long-term effects of Gray's early marriage to Inge Sorensen. Interviewing friends he extracts quotes and comments, both good and bad, with considerable skill. "He's the nicest man I've ever met - I just couldn't take the drinking," remarks Bethsy Gray, one of Gray's former lovers, who tolerated his eccentric behaviour for years, but who found that when her own son became ill with cancer, Gray failed to offer any support: "It was almost as if he didn't understand what was happening." (The two, remarkably, remain friends.) Gray's current and long-suffering wife, Morag McAlpine, Glass fairly reports, "sees faults where others see only the Gray-who-can-do-no-wrong". Pride of place in the personal comments, though, goes to Rosemary Hobsbaum's remark on Gray's eczema: "You used to have to take a Hoover to the floor when he left the room: he neglected himself."

But the real gems in Alasdair Gray of course come from Alasdair Gray. "When Alasdair has been concentrating on work," remarks Glass, "and is given a good opportunity to speak about it, listening to him is like opening up his brain and watching the contents spill gloriously on the table." The book includes plenty of glorious Gray brainspill in the form of reported conversation, letters and miscellaneous remarks. "I've found your life to be a lot of fun," says Glass. "The point is ..." replies Gray, "IT DIDN'T SEEM LIKE MUCH FUN AT THE TIME!"

"Alasdair will only be appreciated when he's dead," writes Glass, getting a little misty-eyed towards the end of what is otherwise a fairly moisture-free account, "and even then it won't be what he deserves. We'll get all the usual sentimental nonsense where commentators will cut out anything uncomfortable in the obituary. The usual parasites will say how 'important' he was." Well, let's just thank God he's not dead yet. And that Glass isn't just the usual parasite.

· Ian Sansom's The Delegate's Choice is published by HarperPerennial