Somebody else has already written a biography of Morrissey. It's taken him 20 years, and the results - frustratingly inconclusive, mischievously misleading, but never dull - are still ongoing. That person is Morrissey.

Songwriters are a coy breed, keen to warn us against reading too much autobiography into their lyrics. Morrissey, you feel, is always enticing us to do so. This makes him the ideal candidate for the kind of treatment Mark Simpson gives him in Saint Morrissey, which takes the simple whos, whats, whens and wheres of its subject's life as read, and instead concentrates on the whys.

Until now, the most prominent book about Morrissey and The Smiths was Johnny Rogan's The Severed Alliance, which so upset its protagonist that he took the author to court (inexplicably, as it was a largely reverent, and well-written piece of work). The Severed Alliance told us what The Smiths did. Saint Morrissey tells us what The Smiths meant, using a mixture of psychoanalysis, lit-crit and guesswork.

If you believe, as I do (having written one myself) that all biography is fiction, then perhaps the only way to arrive at the truth is to make the reverse journey, and examine instead the fictions - the art - of the subject. Or, as Simpson puts it: "To get to the melancholic heart of Morrissey's condition, to get inside the wasteland of his head - or his bed - there is only one thing you need to do. Listen to him."

I've never met Mark Simpson, but feel that I know him. I was the World's Biggest Smiths Fan, the only soul in my small town who understood. Just like all the other World's Biggest Smiths Fans, in their own small towns. And just like Mark Simpson, in his slightly larger town (York).

I saw The Smiths twice, the first time in Cardiff on my 17th birthday, the second time in Chippenham on a school coach trip, and stood transfixed in the middle of the whirling bodies, so static that Morrissey looked at me and raised an "are you OK?" eyebrow. (The fact that my hero had looked at me sustained me for years.) They were, to put it crassly, my Beatles.

After their demise, I followed Morrissey's solo career through the (brilliant) Viva Hate album, but lost interest circa the lazy Kill Uncle, and had given up more-or-less completely by the time he was releasing songs with groaningly unfunny titles like "Dagenham Dave" and "Roy's Keen". Simpson, however, has followed his every move, his every release and his every utterance, and is alive to every nuance, super-sensitive not only to the primary meanings, but to the secondary and tertiary meanings. (He corroborates my theory that "Shoplifters Of The World Unite", released at the height of Clause 28, was intended to be read as "shirtlifters".)

Mark Simpson is gay ("the gay anti-christ", as he was once called by Vogue). When I discovered this, alarm bells started to ring. You see, the suggestion that Morrissey might be gay - and that "this was always the point, if you only chose to decode it" - would, for me, invalidate almost the entire worth of The Smiths.

Obviously, sly hints of homosexuality abound in The Smiths' oeuvre, and if "This Charming Man" isn't about a gay encounter, it's difficult to know what it is about. Furthermore, Morrissey's later fetishisation of rough boys - skinheads, boxers, gangsters, footballers - doesn't require much decoding, and the chorus of "Speedway", possibly Morrissey's finest solo song, is crying out for a closet-door-ajar interpretation: "All of the rumours keeping me grounded / I never said that they were completely unfounded..."

Simpson catalogues all this "evidence", and more. However, he does not pursue this agenda too far, aware that if Morrissey were "just" gay, in a decade which had no shortage of gay pop stars, he would not have been nearly as interesting as he was. Morrissey presented another way of being a man - sensitive, literate, articulate, and above all, asexual - which had nothing to do with existing hetero/homo archetypes. "While the songs are steeped in 'homoerotics'," Simpson writes, "and connote all kinds of queerness, they never denote "gayness" and therefore never exclude those who have not made that identification - which, of course, is almost everyone."

After all, Morrissey declared himself celibate (as well as drugless and teetotal), which instantly made all other bands, with their groupies and their booze and their drugs, look like soiled, discredited, impure slags. This was, in context, an act of heresy. The 1980s were a decade when sex was an imperative, not a choice. The decade when George Michael sang "Sex is natural, sex is good, not everybody does it, but everybody should." Celibacy, therefore, was a perversion, an evil. In celebrating it, Morrissey paradoxically made himself wildly sexual, and his appearance and demeanour - bared nipples, fondled quiff, "Marry Me" written on his chest - eroticised sickly, skinny white manflesh, playing havoc with the hormones not only of his female fans, but of his adoring male ones too.

Simpson has a persuasive way of building an argument, and at one point even makes a case that Morrissey is actually a woman (specifically, a Northern woman, exhibiting "a certain intensity mixed with a certain breeziness, a certain desperation mixed with a lot of self-irony").

He perhaps goes a little too easy on Morrissey's clumsy, playing-with-fire approach to race relations ("Bengali In Platforms", "Asian Rut", "National Front Disco", the Union Jack incident, the pronouncement that "all reggae is vile" and that to get on Top of the Pops "one has, by law, to be black"), pleading defences on Morrissey's behalf which the man himself has been reluctant to give.

Simpson also has more time than I do for the cosy observational comedy of Alan Bennett and Victoria Wood, which inspired Morrissey to write lame elbow-in-the-ribs lines like "I lost my bag in Newport Pagnell" (as though the words "Newport Pagnell" were somehow inherently funny).

Every now and then, his style is a little too gossipy, pun-reliant, and overly keen on alliteration (like Burchill or Lette), but to criticise this too harshly seems slightly underhand, like mocking someone for having a Yorkshire accent. Overall, Simpson writes with enough panache to make most of his peers toss their laptops into the waste disposal and weep. Take this passage, on Morrissey's contradictory relationship with the USA:

"Above all, Morrissey's 'loneliness' is quintessentially American: all Americans are strangers in their own land... The 'United States' is a big, draughty, empty place without enough history or public houses or fish and chip shops to go round, and so Americans wrap themselves in the flag, hug the Cross, huddle on the sports field, or religiously attend the movies... They invented popular culture and consumerism to keep them company, and have very generously exported this form of canned loneliness around the world."

This is the kind of writing which jolts you into raising your own game, kicks you in the posterior with steel-capped boots. Which is, naturally, a tribute not only to the biographer, but to his subject: Simpson has, throughout his life, felt the steel toecaps of Morrissey in his own.