NW, By Zadie Smith

Issues of identity and belonging are at the heart of this busy, densely populated but sombre novel

In Zadie Smith's novels there's often the sense that something critical is missing. Their brilliant parts don't quite add up to a convincing whole. Her fourth shows her virtues – a keen ear for dialogue and deft characterisation. But despite the geographically specific title (NW is a postcode area in north-west London), she does not evoke much of a sense of place, and it's initially hard to discern a plot through the flurries of half-hearted experimentalism (a page about an apple tree laid out to look like an apple tree, for example). Most disconcertingly of all, she seems to have lost her main asset as a writer, her eager comic tone.

Leah Hanwell is disturbed from a back-garden reverie by an urgent ring on the doorbell. A young woman is collapsed in the doorway: "I need some help. I've been to every fuckin door – please. Shar – my name is Shar. I'm local. I live here. Check!" Leah, strangely attracted, gives the waif money for a cab to the hospital. "PAY YOU BACK. GET MY CHEQUE TOMORROW, YEAH?" Though their paths subsequently cross several times, it seems Leah's generosity, as everyone smugly tells her, has been misplaced.

For an awful moment it seems as though Smith is playing the Amis/McEwan game, kicking off a paranoiac storyline about a clash of cultures and classes. But her sense of class is more nuanced than theirs. Or perhaps simply more modern. Leah is a wised-up local herself, though she soon finds her old street smarts count for little in the context of modern gang etiquette.

Red-haired, Celtic Leah is married to gorgeous Michel ("no offence, but for the women in our community, in the Afro-Caribbean community, no offence, but when we see one of our lot with someone like you it's a real issue"). Brisk exposition bluntly informs the reader that on first meeting they "had anal sex before they had vaginal sex". Is there such a thing as overshare with imaginary characters?

Before we have time to ponder that, the focus of the narrative abruptly sweeps from Leah and Michel's problems to a day in the life of Felix, an amiable ex-drug dealer now trying to go straight, or at least straighter. The 60 or so pages in which Felix buys a second-hand car, chats with a kindly old neighbour and attempts to shake off a posh lover (and former customer) are the best in the book. Annie, the brittle heiress, is particularly deftly drawn, although there's something a touch try-hard about the laborious set up of a joke about her neighbours, a "severely dressed Japanese woman" and a "lanky Frenchman, in parodic red braces". They are, inevitably, dubbed "Jules et Kim".

Felix duly dispatched, we take up with Keisha Blake, who is Leah's best friend. There's still not much clue as to what all the jumping around is for. This section, the longest in the book, takes the form of 185 numbered paragraphs detailing the women's lives from the age of four to the present day. Keisha, who reinvents herself as Natalie when she studies to become a lawyer, is gradually revealed as the book's central character, and her sense of alienation from her background, and of inadequacy despite all her money and success, presents us with the sharpest and bleakest observations of modern life.

Keisha/Natalie camouflages her own uncertainty with an urge to interfere in the lives of others. Her marriage is shaky; she and city executive Frank are "two silent enemies shepherding children to their social appointments". Accordingly, Keisha obsesses about Leah's childless marriage to Michel (the anal sex may be relevant after all), and worries about their childhood friend Nathan Bogle, now a dazed street soldier.

In the fleshing out of all these lives there are flashes of the old humour and snap, and a willingness to get amusingly down and dirty with sex scenes. (The one between Felix and the menstruating Annie is reminiscent of Erica Jong.) In didactic mode Smith can come on like George Eliot ("It is perhaps the profound way in which capitalism enters women's minds and bodies that renders 'ruthless competition' the basic mode of their relationship with others"), and then she throws in spot-on contemporary dialogue: "Yeah, well, till you have kids you can't really chat to me, Keisha, to be honest."

The final section, "Crossing", brings these threads together. For all the richness of the detail and the pleasing bounce of the sentences, NW is a more sombre, less energetic novel than we've come to expect from Zadie Smith. In the last line, Natalie again becomes Keisha, "disguising her voice with her voice".

Can you travel far from your roots and still be authentic, still speak with a true voice? It's a question the international literary superstar from Willesden is well placed to explore.