This is a wonderful novel. JK Rowling's skills as a storyteller are on a par with RL Stevenson, Conan Doyle and PD James. Here, they are combined with her ability to create memorable and moving characters to produce a state-of-England novel driven by tenderness and fury. The vacancy of the title occurs when a local councillor, Barry Fairbrother, dies while in office. This is the key event that triggers the action in the superficially idyllic West Country village of Pagford, whose well-heeled inhabitants are at war with the nearby council estate, The Fields. "Nearly two-thirds of The Fields dwellers live entirely off the state and a sizable proportion pass through the doors of the Bellchapel Addiction Centre." The novel charts the struggle between "old", respectable England and the "new" underclass of the estate and it revolves around who will get the vacant council seat. It is a fierce battle  in language, in tone and in its urgency.

It is not too difficult to see certain similarities with Harry Potter. (It would be very odd if that were not the case.) Harry Potter came out of the depths of Rowling's imagination and is in the tradition of portal novels  Alice down the rabbit hole, the children finding their way to Narnia through the wardrobe and Harry through Platform 9¾ at King's Cross. Pagford, the ideal English village, is another "other" world and once you pass into it you are under its spell.

This time, however, it is grounded in current realities  drug addiction, racism, schoolboy bullying as well as the usual stiletto snobberies around the ruined abbey and the village green. Just as Rowling, in her Potter series, got liftoff from Jennings and Tom Brown and other school stories and turned them into something far darker, grander and universally compelling, so here she takes the quiet English village thought to be the copyright of Miss Marple and rebirths it entirely without caricature or wearisome stereotypes. It's a place where the fight between good and evil, between love and despair, never ceases.

As in the Potter series, even some good characters can have a streak of badness in them. Harry is a Horcrux (a vessel containing a fragment of the evil Voldemort's soul); in this novel Parminder, a virtuous doctor, is blemished by her wicked undermining of her youngest and least able daughter.

But above all, as in the Potters, there is the final redeeming grace of love. Severus Snape, who has a lifelong commitment to Harry, protects him because of his unrequited love for Harry's mother. The love in this book is for Barry Fairbrother, who was born on the unspeakable Fields estate but then moved into Pagford, a good man with a mission to integrate the children of The Fields with this privileged village. His chosen instrument for this task is Krystal Weedon, a hardcore resident of The Fields and a stunningly vivid creation. Krystal's background is utterly desperate: she has no idea who her father is; her mother is a drug addicted prostitute; her family lives in squalor. One of her dearest wishes is that they might afford to have a television set and she spends much time at school lying about the programmes she hasn't seen. One of her new friends from Pagford, who wants to be like Krystal, describes her as "funny and tough, impossible to intimidate, always coming out fighting".

When posterity wields its cleansing sword, I suspect that, just as the Harry Potter series will go on being read and cherished, so will the story of Krystal Weedon. Barry includes her in a girls' rowing eight at the comprehensive school. This is the first time anyone has taken any positive notice of Krystal and for a brief moment she thrives. Barry's goodness hovers over the book and becomes its moral touchstone, a counterpoint to the nastiness elsewhere. There's a vile character called Simon, whose domestic reign of terror over his wife and two sons echoes the fascist stratum throughout Harry Potter. Another villain is a school bully who (verbally) tortures a young Asian girl and drives her to extreme self-harm. Then we learn that he is adopted, which tempers our judgment. Screens are pulled away again and again and someone who seems simple to judge  and Rowling's characters provoke judgment  becomes more complex.

The ability to create characters who move you so much that you want to shout aloud "Don't do that!" or "Don't say that!" is rare. Krystal Weedon moved me to laughter, admiration, distaste, anger and finally, in her great battle against such unfair and immense odds, to tears. Rowling is expert in tracking through a trivial plot line (the stealing of a watch) and making it play its part along the way in the bigger game. She is a compelling organiser of debate. The arguments put forward by the Pagford loyalists against what they see as the evasive sullen rabble from The Fields are laid out with conviction, as is the opposing view.

But at other moments, she erupts into partisan rage. There's a scene where Parminder the doctor, a councillor, listens at a meeting to the "extravagantly" fat chairman condemn those in The Fields for the expense they cause to the NHS. Parminder, unwisely but uncontrollably, then spells out what this man's total refusal to lose weight has cost the NHS in heart operations, expensive drugs, constant appointments and so on. There are moments, too, of utterly bleak comedy, such as when Terri, Krystal's mother, whose council house is a rubbish dump, attempts to defend her patch against the predators from the social services.

One mark of Rowling's determination to get to the nerve of reality in this book is her vigorous use of industrial language. Fs are everywhere, Cs are not withheld. But it never feels gratuitous.

Rowling has spoken of the sense of risk in embarking on this novel. The Harry Potter series must have been a tough act to follow. What she wanted to do here, I guess, was to seize on the world we can all see without going through Platform 9¾. She has done that to stunning effect.

Featuring grimy casual sex, self-harm, rape and heroin addiction  not to mention vicious parish politics  JK Rowling's first novel for adults represents a decisive break from her back catalogue. But if The Casual Vacancy is ambitious in its scope and themes, it is determinedly unadventurous in its style and mode. It's a book that wrestles honourably and intelligently with big moral and political questions, but does so in a slightly clunky and convention-bound way.

Rowling relies on stock situations and verbal clichés; if you're irritated by important episodes being telegraphed with phrases such as "But then came the hour that changed everything," then this is probably not the novel for you. But equally, it offers something that more stylish, highbrow fiction often doesn't or won't: a chance to lose yourself in a dense, richly-peopled world.

This is a traditional, somewhat retro English novel. Set in the "pretty little town of Pagford", in the West Country, it is a study of provincial life, with a large cast and multiple, interlocking plots, drawing inspiration from such writers as Trollope, Gaskell and, perhaps most of all, George Eliot: the tone is empathetic, but censorious and slightly didactic, using the plot to show that wickedness rebounds on both the wicked and the virtuous, leaving us all sadder and wiser at the end. The only obvious parallels with the Harry Potter books are that, like them, it focuses on teenagers, and is animated by a strong dislike of mean, unsympathetic, small-minded folk. The inhabitants of Pagford  shopkeepers, curtain-twitchers, Daily Mail-readers  are mostly hateful Muggles, more realistic versions of the Dursleys, the awful family who keep poor Harry stashed in the cupboard under the stairs. The book has already been dubbed Mugglemarch.

Behind its tourist-friendly façade  the hanging baskets, the war memorial, the scrubbed cottages  Pagford is of course a hot-bed of seething antagonism, rampant snobbery, sexual frustration and ill-disguised racism. "Old Pagford", for instance, finds it hard to forgive the "brownness, cleverness and affluence" of its spiky Sikh GP. The plot is set in motion when, on the third page, its hero, Barry Fairbrother, falls down dead in the car park of the "smug little golf club"; "smug" and "complacent" are words that crop up again and again.

His death creates a "casual vacancy" on the parish council, and the forces of darkness, led by Howard Mollison, the obese delicatessen owner, see their chance to parachute in one of their own. Barry, a man of "boundless generosity of spirit", had been the main opponent of their plan to reassign the Fields, a run-down sink estate, to the district council of the nearby city, Yarvil  thereby off-loading responsibility for its drug-addled inhabitants, and driving them out of the catchment area for Pagford's nice primary school. The election heats up when scurrilous but accurate accusations, posted by "the Ghost of Barry Fairbrother", start appearing on the council website.

The other main driver of the plot is Krystal Weedon, the teenaged daughter of a chaotic heroin addict from the Fields, outwardly aggressive but actually doing a good job of single-handedly raising her neglected little brother. Barry, who grew up in the Fields before making good and moving up the hill to Pagford, had been a sort of mentor to her. Less altruistically, Krystal is being pursued by Fats, the clever and mildly sociopathic son of the deputy headmaster and the school counsellor. He doesn't think about her much as a person; he thinks of "those splendid breasts, that miraculously unguarded vagina".

Judging by the early reviews, people are shocked that the author of Harry Potter could have written something as bleak as the Weedon family. But these sections of the book are a little too laborious and programmatic to be truly harrowing: like a detective series dutifully dealing with "social issues", it seems to come at the underclass story via what we already know from journalism, or from social workers, rather than inhabiting it from the inside. Rowling is impressively unsentimental, and has clearly done her research. But though there are some viscerally horrible touches, these passages don't feel fully alive. Perhaps that's partly because the Fields characters are handled with the tweezers of old-fashioned literary convention: whereas the others speak in rounded standard English, they use a kind of generalised, Dickensian lower-order-speak, that owes more to written convention than anything real: "I takes Robbie to the nurs'ry"; "Tha's norra fuckin' crime"; "No, shurrup, righ'?"

Personally, I found it far more convincingly bleak in its more satirical sections dealing with the attitudes of the supposedly tolerant middle-classes, and their pinched and loveless relationships.

Probably the best bits of all are those featuring teenagers, and particularly teenage boys. Fats, an adolescent existentialist ruthlessly searching for "authenticity", is a memorable character: he's made the unnerving discovery that, in his age group, being immune to embarrassment is almost like a super-power. (Rowling is said to have drawn on her own experiences growing up in genteel West Country villages, where she was reportedly moderately unhappy, listening to the Smiths and feeling desperate to escape.)

By and large, as Fay Weldon put it back in 2003, Rowling writes "readable, saleable, everyday, useful prose". But there is a persistent sense in The Casual Vacancy that the language is being strained for fine effects, and not doing quite what Rowling wants it to do. The GP, we are told, "hated sudden death", which seems an odd statement to make.

The metaphors and similes often slip away from her. One character's sexual performance is "as predictable as a Masonic handshake"; another clings on to her partner like "an aggressive and threatening barnacle"; the news of Barry's death radiates out, "halo-like", from the hospital. You can see what she's getting at in all three cases. But Masonic handshakes aren't predictable unless you happen to be a Mason  they're mysterious and faintly sinister; barnacles aren't threatening or aggressive; and haloes don't radiate outwards  they sit on saint's heads, or surround planets.

For Rowling's fanbase, this is probably beside the point. They'll be in it for the plot, which is big, engrossing, and runs like clockwork. Like the Potter novels, the book is extremely efficiently organised beneath the busy surface. True, the narrative requires a fair amount of artificial contrivance, is a little a predictable, and lurches into melodrama in the final straight  but this kind of story probably demands that. More problematic for the fans, I suspect, is that The Casual Vacancy has none of the Potter books' warmth and charm. It's a pretty sour story, in which all the characters are either fairly horrible, or suicidally miserable, or dead. Even Barry is a terrible disappointment to his wife.

People have come up with all sorts of explanations for Harry Potter's popularity with grown-up readers, from the obvious  the escapist attractions of the minutely invented fantasy world  to the grandiose: western adults are stuck in an eternal adolescence. But one of them, I suspect, is that people still enjoy reading about good people, and seeing them rewarded  something that more respectable novels seldom offer these days. If that's the case, then Rowling might see a backlash over the next few months.

They call it "denial marketing": the process whereby the contents of JK Rowling's books are guarded like the crown jewels until publication day. It made sense with Harry Potter, when the world and his dog wanted to know what had happened to the boy wizard and his dastardly foes. But it creates a slight anti-climax in the case of The Casual Vacancy, a novel concerning a parish council election in a small West Country town.

There are some superficial excitements here, in that the younger characters get up to things that Harry probably never dreamed of: taking drugs, swearing, self-harming, having grimy casual sex, singing along to Rihanna. The new book contains regular outbursts of four-letter words, along with the memorable phrase "that miraculously unguarded vagina"  which, leaked in a pre-publication profile, has caused a flurry of jokes on Twitter about Harry Potter and the Miraculously Unguarded Vagina.

Generally, though, The Casual Vacancy is a solid, traditional and determinedly unadventurous English novel. Set in the "pretty little town of Pagford", it is a study of provincial life, with a large cast and multiple, interlocking plots, drawing inspiration from Elizabeth Gaskell and George Eliot. The only obvious parallel with the Potter books is that, like them, it is animated by a strong dislike of mean, unsympathetic, small-minded folk. The inhabitants of Pagford  shopkeepers, window-twitchers, Daily Mail readers  are mostly hateful Muggles, more realistic versions of the Dursleys, the awful family who keep poor Harry stashed in the cupboard under the stairs. The book seems doomed to be known as Mugglemarch.

Behind its tourist-friendly façade  the hanging baskets, the war memorial, the scrubbed cottages  Pagford is of course a hot-bed of seething antagonism, rampant snobbery, sexual frustration and ill-disguised racism. The plot is set in motion when, on page five, its hero, Barry Fairbrother, falls down dead in the car park of the "smug little golf club". His death creates a "casual vacancy" on the parish council, and the forces of darkness, led by Howard Mollison, the obese delicatessen owner, see their chance to parachute in one of their own.

Barry, a man of "boundless generosity of spirit", had been the main opponent of their plan to reassign the Fields, a run-down sink estate, to the district council of the nearby city, Yarvil  thereby off-loading responsibility for its drug-addled inhabitants, and driving them out of the catchment area for Pagford's nice primary school. The election heats up when scurrilous but accurate accusations, posted by "the Ghost of Barry Fairbrother", start appearing on the council website.

The Casual Vacancy has all the satisfactions and frustrations of this kind of novel. It immerses the reader in a richly peopled, densely imagined world. Rowling has reportedly drawn on her own mildly unhappy West Country childhood, in a village outside Bristol and then later outside Chepstow. The claustrophobic horror is nicely done: everyone knowing everyone; Howard, scheming from behind his hand-baked biscuits and local cheeses. Rowling is good at teenagers, particularly boys, and unhappy couples. The book has a righteous social message, about responsibility for others, and a great big plot that runs like clockwork; like the Potter novels, it is efficiently organised beneath its busy surface.

On the other hand, the novel is very much the prisoner of its conventions. Rowling's underclass characters are not bad, considering they were put together by the richest novelist in history, but it's a pity that they all use a kind of generalised, Dickensian lower-order-speak, that belongs more to literary custom than anything anyone ever says: "I takes Robbie to the nurs'ry"; "Tha's norra fuckin' crime"; "No, shurrup, righ'?". The plot is often predictable; it requires a large helping of artificial contrivance; and it lurches into melodrama in the final act. The rules probably require this, and it all rattles along nicely enough, but it leaves a slight sense of disappointment.

No one, I suspect, reads Rowling for the beauty of her sentences but there is often a sense here that the language is not quite doing what she wants it to do. One character, we are told, "hated sudden death". Who doesn't? The metaphors regularly run away with her. One character's sexual performance was "as predictable as a Masonic handshake". What's predictable about that?

The Casual Vacancy is no masterpiece, but it's not bad at all: intelligent, workmanlike, and often funny. I could imagine it doing well without any association to the Rowling brand, perhaps creeping into the Richard and Judy Book Club, or being made into a three-part TV serial. The fanbase may find it a bit sour, as it lacks the Harry Potter books' warmth and charm; all the characters are fairly horrible or suicidally miserable or dead. But the worst you could say about it, really, is that it doesn't deserve the media frenzy surrounding it. And who nowadays thinks that merit and publicity have anything do with each other?

The Casual Vacancy by JK Rowling. £20, 503pp