A long long time ago, back in the early 1990s, there were only three novels about Edinburgh. There was Robert Louis Stevenson's The Strange Case of Dr Jekyll & Mr Hyde (which wasn't actually set in Edinburgh at all), there was James Hogg's Private Memoirs and Confessions of a Justified Sinner and more recently Muriel Spark's The Prime of Miss Jean Brodie. The Scottish capital had plenty of other books to its name but those were the only three that mattered, the ones that knocked through all of Edinburgh's lovely false fronts to the far more interesting things behind.

Forty miles away, there were writers falling over themselves to talk about Glasgow: James Kelman, Alasdair Gray, Jeff Torrington, AL Kennedy, Janice Galloway. Somehow, Edinburgh's great rival just offered more to describe  more people, more history, fewer layers to scrape off between appearance and truth. Glasgow was cool, and whatever Edinburgh was  beautiful, vertiginous, cold  it had never in its whole life been cool.

And then in 1993, a book called Trainspotting was published by an unknown council worker called Irvine Welsh. Trainspotting told the story of another Edinburgh entirely; not the pretty neoclassical bit in the middle, but the schemes and estates beyond the invisible Pale  places like Saughton and Niddrie and Sighthill which had been used by the Tories to test out contentious new ideas: closing the car plants, bringing in the poll tax, flooding the neighbourhoods with cheap high-grade heroin By the time Trainspotting came out, Edinburgh had stopped being the Athens of the North and become instead the Aids capital of Europe.

If Welsh had set his novel in Glasgow, there would have been a brief fight about swearwords and then silence. Because he set it in Edinburgh, Trainspotting went off like a detonation. It was written in a thick Leith accent and it told the stories of a bunch of neds and schemies on the hunt for dole, sex and junk. But Trainspotting's real brilliance was the zest and joy and sheer black-hearted energy of its writing. Now, nearly 20 years later, it's difficult to imagine Scotland without the psychopathic Begbie or Bond-obsessed Sick Boy, or Renton's echoing lament: "Some hate the English. I don't. They're just wankers. We, on the other hand, are colonised by wankers"

Britain loved Trainspotting, Scotland loved the spectacle of Edinburgh with its knickers off, and the city itself remained ambivalent. On the one hand, the revelation that it had as many issues with sectarianism and STDs as the wild wet west irritated the New Town enormously; even now, the official Edinburgh Unesco City of Literature website pays Trainspotting the great compliment of completely ignoring it. On the other, most people soon realised that Welsh had somehow pulled off the trick of conveying a pungent anti-drugs message while simultaneously making Edinburgh look interesting. Suddenly the place was full of Fettes boys hanging out at the Foot of the Walk and talking radge with their swedgin and barry.

Anyway. All that said, what exactly is the point of Skagboys? Welsh can never be unknown again. His writing can never shock like it once did. No prequel or sequel can have the impact of Trainspotting. Ecstasy  his 1996 follow-up  didn't exactly tank, but nor did it reach anywhere near Trainspotting's giddy heights.

And first impressions aren't great. Skagboys is long: 548 pages in hardback. There are a lot of characters and too many voices. Sometimes, when Welsh slips back from Edinburgh to English  as he does with Renton's parents  the deficiencies in his fiction become obvious. Worst of all, there are signs that he's using Skagboys as a teaching aid. Brief socio-historical pass notes are included every couple of chapters for the benefit of anyone too young, too posh or too English to have noticed what was happening to Scotland during the 1980s. Optimum laboratory conditions, in other words, for a disastrous read.

Except that Skagboys isn't. We start out back in the days when Mark Renton is clean and reading for joint honours at Aberdeen Uni. He and Sick Boy take their first fix. Renton's brother Wee Davie dies. The heroin begins to take hold. His family begin to disintegrate and the good girls fall away. He drops out of university and goes to work on the boats. "Schopenhauer was right," Renton thinks: "life has tae be aboot disillusionment; stumbling inexorably towards the totally fucked."

For anyone who loved Trainspotting first time around there's something deeply cheering about returning to Welsh's world. "Are you sexually active?'' asks the woman at the Aids clinic. "Usually, aye, Keezbo goes, no gittin her at aw  but sometimes ah jist like tae lie back wi a bird oan top"

For those who want to be shocked, there's plenty of provocation  Renton tossing off his "spasticated" brother, puppies down rubbish chutes munching on aborted foetuses, a memorable incident involving budgies and a mastectomy  and plenty of perfect moments: Renton torn by "resentment and tenderness" beside his grieving parents, Begbie raging at others for admiring his singing voice, Renton and Sick Boy wound round each other like bindweed.

And many of Welsh's wider points are well made. His "Notes on an Epidemic" include a couple of monthly lists of reported HIV-positive cases: 39 names, each with their terrible case histories summed up in a sentence. "If being Scottish is about one thing, it's aboot gittin fucked up," Renton explains. "Tae us intoxication isnae just a huge laugh or even a basic human right. It's a way ay life, a political philosophy."

And then things go slack and you can sense Welsh's concentration wandering. Twenty years later, he's too far from a world he was already distanced from when Trainspotting came out. Besides, part of the problem of writing about drugs is that almost by definition, every fix gets a little less interesting: heroin, like happiness, starts to write white. For hardcore enthusiasts, Skagboys' more measured pace and broad overview is a treat. For everyone else Trainspotting said it all, and said it better.

There are basically two types of Irvine Welsh novel. There are the deeply felt and vividly evoked stories about young men going to the bad on the mean streets of Leith (Trainspotting, Glue). Then there are his silly, sick ones, whether the genre is tapeworm-infested police procedural (Filth), evil-double gothic (The Bedroom Secrets of The Master Chefs) or psychedelic thalidomide revenge fantasy (Ecstasy).

This is a simplification: the desire to shock runs deep in Welsh, and all his books are at least slightly sick and sensationalistic. But when it comes down to brass tacks, Welsh has one great story to tell; and much of the rest of his career has been spent failing to find another one in ever more desperate and revolting ways, apparently feeling that he's letting the side down if there isn't a haemorrhoid-scratching scene or a ruptured penis or child being dismembered every few pages.

I was really looking forward to Skagboys, which falls clearly into the first category. The prequel to Trainspotting, it is billed  like most of his novels  as his best since then. It begins in 1984, and it follows Renton, Sick Boy and Spud as they become heroin addicts, and Begbie as he graduates from Leith bully to fully-fledged Edinburgh-wide psycho. The prologue, in which Renton joins his father's union on the picket line at the Battle of Orgreave during the miners' strike, sees Welsh at his sweary, eloquent best. It kicks off in convoluted standard English before liberating itself into broad housing scheme Scots. Renton is beaten up by the police, and left with back pain and a sense that there's no future: "Ah'm thinkin that we've lost, and there's bleak times ahead, and ah'm wonderin: what the fuck am ah gauny dae wi the rest ay ma life?"

At this point, the book seems to be shaping up as a blunt but powerful anti-Thatcherite epic. Spud is laid off from his job as a removals man. Renton finds himself doing de-skilled carpentry work for "the kind ay small businessman Thatcher loves; a grasping, spiritually dead, scab-minded cunt". There are mini-essays about the Tories' failure to devolve power to Scotland, and about the spread of HIV in Edinburgh; there are sub-plots set in the pharmaceutical factory from which high-grade heroin is flooding on to the streets. But ultimately, it's a more confused and personal book than that.

Renton has all the chances  a place at university, a lovely girlfriend  but blows them all, owing, it seems, to his torment over his "spazzy" younger brother, and to something like an identity crisis: "That's ma problem; ah'm too fuckin poncy tae be a proper Leith gadgie n too fuckin schemie tae be an arty student type." In the end, the blame for the gang's wasted lives is shared between Thatcher, class, screwed-up families and something else: original sin, maybe.

Coming back to Trainspotting nearly 20 years after it was published, it still seems like one of the more interesting British novels of recent decades. It's a sort of hellish social comedy, in which a whole subculture is artfully compressed into 44 fragmented chapters. You sense that Welsh knows the terrain like the back of his hand; that he knows where every minor character went to school, and how they got their nickname. Along with a great ear for dialogue and local detail, Welsh has a powerful wider political story to tell: how the country's industrial communities were hollowed out in the 1980s ("the substitution of drugs for jobs in the poorest parts of Britain", as he has put it). But he tells it in an exhilarating style that makes the traditional social realist model for such stories  think of Ken Loach or early James Kelman  look earnest and plodding.

Perhaps Welsh was able to do all this because he had in fact written an entire unpublished novel's worth of background. When the sequel, Porno, came out 10 years ago, he said that he had written "a huge amount more of Trainspotting than went into the book, but I didn't want to rehash that". Well, now he's done exactly that, and the result is Skagboys. Presumably this is why it reads like somebody's interesting but confused first novel: nakedly autobiographical, stylistically uneven, with some fascinating passages and a fair amount of earnest and plodding social realism. Welsh appears literally to be finding his voice, ranging from student-pretentious ("the lines from that classic Dylan Thomas poem resonated in her") to Penguin Classic ("Ronnie turned to George, estimating that his younger brother was more likely to be a peer of this boy who had disgraced their sister"). Only in some sections does he hit the confident, precise tone of Trainspotting, in which casually deployed official English collides with the vernacular: "You can operate fae the purest of motives but some fuckers will eywis misconstrue it to fit their ain twisted agenda."

Skagboys is also just too long: it essentially goes over the same ground as Trainspotting, yet it's about three times the size. There is the same run of highs and lows, skag hits and funerals. Sick Boy shags and mistreats his way through the book, while Begbie batters and stomps and chibs. There are even reruns of famous passages from the previous novel, including a minor-key reprise of Renton's "choose life" rant about how he doesn't want a nine-to-five and a mortgage. The bits that deviate from the old routines often seem the most uncertain. There's a slack, meandering section set in a Hackney squat. And Renton's happy student days are a little insipid  he even goes InterRailing!

For all that, there are many unforgettable episodes, such as the visit to a squalid shooting gallery, where one large hospital syringe passes round the room from junky to junky, like the angel of death. Even at his weakest, Welsh performs the mysterious feat of making you think that his characters are real. But by the time, a few pages from the end, that Renton is heard saying: "it's no like some famous cunt's gaunny come along and make a film ay our lives, is it?", the reader has a vivid sense of a great talent tamely revisiting his glory days.