Zadie Smith's new novel is oddly divided between confidence and indecision. The confidence is easy to understand, given an enviable alignment of talent and readership, which offers the possibility of being faithful to roots without being bound by them, ignoring the old rules about minorities and the mainstream, and politely rejecting the role of poster girl for post-ethnicity. The indecision is harder to account for. Uncertainty keeps on cracking the pavements and makes for a stumbling journey through the streets of the book.

"NW" is north-west London, though the focus is tighter, largely on Willesden (south London being no more relevant than Tierra del Fugo). The main character in the first section is Leah Hanwell, a Willesdener of Irish descent now in her mid-30s, brought up on a council estate with a dodgy reputation, still living nearby though in relative comfort. Leah has a philosophy degree but works ingloriously in an office where powerlessness is dressed up in the language of empowerment. She is warmly teased  with an undercurrent of real resentment  by her female, African-Caribbean co-workers (and they're all female and African-Caribbean) for having laid hands on a treasure that rightly belongs to their community  her husband Michel, a francophone black man hoping to earn a better living from online investing than he has from hairdressing. The only flaw in the marriage is that Michel wants children and Leah does not, though she's never said so.

There's a lovely rippling effect over the opening pages, with Leah's thoughts and surroundings enriching each other, rather in the manner of Ulysses. It begins to look as if Joyce will be the patron saint of this novel, as Forster watched over the previous one (On Beauty). It's a style that doesn't bed down, though. The short length of sections works against the sense of total immersion that suits stream-of-consciousness writing. There are still modernist moments, but they take the form of minor flourishes, such as Leah's wandering thoughts being presented on one page in the shape of a tree, and a monologue from Michel (to which she's barely listening) on the next.

Leah may be a native Londoner, but she has her naive side, and falls for the standard hard-luck story (mother desperately ill in hospital, no money for a taxi) told to her by a distressed woman knocking at the door. She offers £30, feeling some sort of rapport, particularly when it turns out that the two of them went to the same underperforming school. Michel and her mother make common cause in scolding her for gullibility, but even when she realises she has been scammed Leah's feelings for the con artist (name of Shar) remain conflicted.

These tendrils of plot and situation could be trained across the trellis of various genres. A woman who wants everything but the baby her man has set his heart on? Chick lit but with deeper possibilities. An encounter with a stranger leading to an invisible criminal underworld very close at hand? London Gothic à la Ruth Rendell, perhaps, but with convincing youth details. Smith doesn't develop these strands, but she doesn't risk cutting them loose altogether.

Meanwhile the tempo slackens, as if a well fitted-out yacht were waiting for a breeze that never comes. Not all the secondary decisions are successful, but at least they get made. One of them is to present dialogue without inverted commas, as Joyce did (he hated those marks, calling them "perverted commas"), using a dash instead. This preference calls for extra clarity when it comes to demarcating the end of speeches. What to make of this, for instance? " I can see the magistrates' court and  a roundabout? Kids, stay close, stay in. It's like walking the hard shoulder on the motorway. Nightmare. Kennedy Fried Chicken. Polish Bar and Pool. Euphoria Massage. Glad we took the scenic route. This can't still be Willesden. Feels like we're in Neasden already." Ulysses taught readers to read sentence-fragments as signals that the barriers between inside and outside, speech and thought, were dissolving, but here the whole paragraph seems to be spoken aloud.

There's an unpredictably changing distance in the point of view as it addresses Leah. The equivalent in a film would be jarring alternation between long shot, two shot and extreme close-up. There's even some wobble in matters of detail. Leah's mother thinks goods in Poundland can be priced at £2.49, and a local chemist's does a brisk trade developing films. At one stage Leah puts a payment on an old credit card from her student days, to prevent Michel from finding out. That's quite a trick, with a card so long expired.

The whole of the first section is defined by its resistance to genre, by what it doesn't want to be. It's like an oddly shaped inner-city park, bounded not only by chick-lit and thriller but by the modernism it aspires to. The touches of dilute Joycean play are less like new ways of looking at the world than mildly adventurous ways of organising a narrative. NW even abuts on the territory of the "Hampstead novel" (Hampstead being geographically close however socially and spiritually distant), that antique dismissive term for novels in which middle-class people alternately gloat and lament over their privileges. Leah's oldest friend Natalie invites her and Michel to dinner parties whose conversation is reproduced as a composite stream of banalities ("Let me tell you about Islam") and food fetishism ("Pass the green beans with shaved almonds"). There's a touch of bad faith here, since successful authors are rarely looking at dinner-party rituals from below the salt. The whole book is oddly queasy about the value of getting on in the world.

In the next section the tone warms up. Inverted commas make a return, like birdless wings after some seasonal migration, bringing with them an immediate uplift in terms of readability. The main character here is Felix Cooper, a recovering addict putting his life back together and rejoicing in a recently established relationship. Encounters with his father and a neighbour sketch in a painful but not hopeless background. The dialogue can't avoid the pervasive non-interrogatives "innit" and "is it", but isn't ruled by them ("to chirps", meaning " to chat up", is lovely). Felix sets out from NW6 to W1 to inspect a derelict sports car owned by a posh boy named Tom, going cheap, but also pays a visit on impulse to Annie, an ex-lover of his based in Soho.

It's perverse to represent Tom's point of view, without the necessary knowledge or sympathy, but Annie is the one privileged character in the book who isn't dead on the page, perhaps because she survives by performing her class status, in the hope that poshness will disguise poverty. The section about Felix's day is certainly the most successful in the book, though it connects weakly with the rest, as if this were a separate project, imperfectly incorporated.

The rest of the book is devoted to Leah's friend Natalie. There's nothing limited about female friendship as a subject, as long as you have confidence in it. But the Leah panel and the Natalie panel simply don't line up  the hinges grind. The time scheme moves past the original dramatic set-up, the entanglement with Shar, as if it had never been important.

This is the section that works hardest to achieve consistency of tone, but the chosen tone is an odd one, of brittle distance. The character is routinely referred to as "Natalie Blake", as if the writer was reminding herself not to get close. Numbered subsections suggest a series of propositions, about marginality, education, privilege, rather than a felt story. Sometimes subsections need a title to clarify an allusion, so that "178. Beehive" establishes, for the reader in need of clues, that the singer being described ("this voice sounded like London") is indeed Amy Winehouse. Similar contortions of reference shroud perfectly ordinary mentions of Friends and The Wire. There's no sophisticated response to the world that excludes irony, but the irony here seems anxious and self-protective. It's in this section, where she works hardest at building a wall between character and reader, that Smith also feels the need to break through it with misjudged interventions along the lines of "You're welcome" and "In case you were wondering " The conflicts within the writer are deeper than the ones she has devised for her characters.

The trailing plot threads aren't exactly tied off, more tucked back in. The real mystery of NW is that it falls so far short of being a successful novel, though it contains the makings of three or four.

 Adam Mars-Jones's Cedilla is published by Faber.

Although Zadie Smith's much-awaited new book is called NW, in honour of the London postcode in which it is set, no one is talking it up as the great metropolitan novel we are all (supposedly) waiting for. Or not so far. I wonder why. You could put this down to its author's gender, of course; most critics seem only to be willing to shove the adjective "great" up against the reassuring name of a man. But in the case of Smith, so acclaimed, and so very clever, I really don't think this is it. We must go elsewhere.

Is it a prerequisite of big London novels that they must contain as many bankers, four-wheel drives and yoga classes as cleaners, double-decker buses and condom-strewn playgrounds? Perhaps. Greatness, in the 21st century, seems to be associated mostly with grand and sometimes slightly improbable oppositions, with what you might call "sweep": high and low, ritzy and gritty, beautiful and ugly. Punctuate your swooping narrative with newspaper headlines  the financial crash! Bombs on the underground! Multiculturalism in crisis!  and talk of brilliance, however fleeting, will soon hover obligingly in the ether.

Smith's novel  sorry to fall back on such an old metaphor  is written on an inch of ivory: a universe away from the roaring, schematic books of her male counterparts. Occasionally, the Technicolor world outside seeps in: one character knows someone who has been marooned by the ash cloud in Iceland; another watches the television news and sees men in suits leaving their shiny offices, cardboard boxes in their arms. But in the main, events, in the biggest sense of the word, are far away. NW's interest is at once more quotidian and more vital.

How, its author wants to know, do people survive in the city? How does it work, this trick of living in almost complete isolation from human beings who eat, talk and sleep only a few feet away? And when your peers disappear, as they are wont to do, where is it that they disappear to? NW is a novel about escape, but one so rooted in Smith's sense of place, not to mention her highly tuned awareness of the infinitely subtle gradations of social class, that there are moments when you wonder if her tunnellers will ever come up for air again.

Leah, Natalie (formerly Keisha), Nathan and Felix all grew up in Caldwell, a Willesden council estate: "Five blocks connected by walkways and bridges and staircases, and lifts that were to be avoided almost as soon as they were built. Smith, Hobbes, Bentham, Locke, Russell. Here is the door, here is the window. And repeat, and repeat." Now they are in their 30s. Nathan and Felix are among the disappeared, in the sense that their lives have bent out of shape (drugs, crime, women). Leah and Natalie are among the disappeared, in the sense that they have moved up, and out (exams, university, employment). Set down here, this sounds didactic, a touch Elizabeth Gaskell. But it isn't. Not at all. Nathan and Felix, constantly in motion and willing to make full use of the various means by which one can numb pain, bewilderment and anxiety, are not happy, exactly. But you know by the attention Smith gives them, roughly a quarter of that which she devotes to Leah and Natalie, that their dispositions lean more easily towards contentment. The girls, on the other hand, are prone to introspection and unease; they notice stuff, piercing details that ruthlessly undercut what feeble foundations they have been able to lay thus far.

Best friends, they measure their lives in terms of others: specifically, each other. "Leah passes the old estate every day on the walk to the corner shop. She can see it from her backyard. Nat lives just far enough to avoid it. Anyway, all meetings happen here, at Nat's house, because why wouldn't they? Look at this beautiful house!" You wouldn't believe the apprehension to be found in the length of a Queen's Park garden; it rises like summer fog.

Is there a plot? Barely. It's a quietly ticking clock, not some monstrous oily engine. The novel opens with Leah, dreaming in a hammock. The doorbell rings, and she rushes, "sun-huddled", to answer it. Outside is a young woman: filthy, smelly, in dire straits (you could write a whole essay on the theme of dirt in NW; Smith is as meticulous when it comes to describing muck as she is cherry blossom). The woman asks for, and is given, a loan  though this will never, of course, be paid back, and it haunts the first section of the book, a reflection of Leah's general hopelessness (badly paid charity work, dreamer husband, rented flat) when set alongside Natalie's apparently replete composure (barrister, rich husband, two beautiful children). We're a long way, now, from the light and hope of Smith's last novel, On Beauty.

There follows an interlude, funny, violent, bulging with the dialogue for which Smith has such an ear; there is a walk-on part for one of the best posh junkies  "I said I'm clean, not a bloody Mormon!"  I've ever come across. This is all for Felix. But then we track back to Leah and Natalie. This time, Smith is going to take them all the way from school, through university, and into marriage. She does this in a series of brief sketches. They're numbered, so I can tell you that there are 185 of them. Pretty much every one is brilliantly written. Her sentences are truly, distractingly ace; she has all of the sass of the young Martin Amis, and none of the swagger. But I worried, sometimes, about form. I get the whole snapshot thing, the attempt to mimic the stopwatch of memory, which returns to us only in hazardous flashes. I understand, too, that cities are, like the trains that rumble through them, all stop-start. On the other hand, it felt desultory, sometimes, as if it had been written in short bursts (and perhaps it was).

I wasn't surprised to find that behind Natalie's cool exterior was a personality altogether more fluid, more febrile. But I thought her secret outlet for this discomposure  I won't spoil things by naming it  wildly unconvincing. There is, too, something uncomfortable about the novel's end, when Smith seems somehow to run out of steam, and both Leah and Natalie retreat shufflingly to a position previously held up by the novel for our disapproval (the idea that people get what they deserve). These are, I think, quite serious flaws. On the other hand, a writer must be allowed to take risks. Smith deals in character, not stereotypes; she couldn't give a fig for box-ticking, for the neatness that publishers, and some readers, seem to crave. This, for those who devour a lot of new novels, will come as a relief. And besides, her wisdom, her wry engagement with all the things (pound shops, vibrators, headbands made of old tights) and all the people (slipper-wearing bums, readers of Grazia, calcified commie postmen) that most contemporary novelists avoid like the plague, is everywhere to see. All of which is a rather long-winded way of saying that the wonderful bits more than make up for the less wonderful, and that you should rush to buy this book before the summer is out.