Zadie Smith’s fourth novel bends but doesn’t quite break under the grinding weight of expectations. Too engaging and intelligent to disregard, too incomplete to satisfy, too shakily structured to stand with its predecessors, NW exists awkwardly in the middle ground, much like its characters, who have grown up in a northwest London housing project but can’t shake free of its influence.

The inconsistencies are confounding, but then, this is what happens when you write something as staggeringly prescient and transcendent as White Teeth when you’re 24, or, a few years later, you reconstruct E.M. Forster’s masterpiece Howards√ End as the gorgeous On Beauty, a wise treatise on art, race, culture and politics. Readers are willing to allow you The Autograph Man — not a bad book, but still a little brother. But they grow less patient when the work represents a step in the wrong direction. Shouldn’t you try narrative experimentation in your first novel or maybe your second? Not now. Not this far down the road. Not when we expect and crave so much.

In many ways, though, NW is pure Zadie Smith; no overused, postmodern style tricks can obscure her talent. Like her earlier novels, NW is a penetrating examination of the cultural, racial and class divisions that separate us. “It’s an island we’re on here. I always forget that, don’t you?” one character asks two strangers on the train. She’s talking about Britain, but Smith is talking about something else: We’re all on an island, yeah? We’re alone, despite spouses and families, to sort out this mess of a life. And no one can help us when past and future collide.

Such dissonance rattles the lives of Leah Hanwell and Natalie Blake, best friends as girls who drifted apart in adolescence (over important issues like Joy Division, Sonic Youth and William Burroughs), stumbled back into friendship during their college years and now maintain an uneasy alliance. In their mid-30s, neither is on sure footing.

Red-headed Leah is married to handsome Michel, who “can’t deny that he is more beautiful. … His skin is very dark and ages more slowly. He has good West African bone structure.” She should not be surprised, Smith points out, that Michel wants to have a child — this is, after all, what people do at this point in their lives — but Leah is surprised, and she is wary. She can’t even cope with a door-to-door scamster the way one is supposed to in this neighborhood (with a forceful blend of outrage and authority).

In Leah’s eyes, Natalie is the one who can handle anything, the one with a perfect life: lovely flat, law degree, wealthy, biracial husband, two adorable children. But Natalie knows different. Born Keisha but determined to leave all traces of her Caribbean immigrant roots behind, Natalie has spent her youth “crazy busy with self invention,” working hard to erase her mother’s fretful religion, her sister’s poor decisions, her own memories of poverty. She has shut out her mother’s refrain that “whatever you did in life you would have to do it twice as well as they did it ‘just to break even,’ ” even as she has dedicated herself to obeying it. And look where it has gotten her! “Mother and child. Look at you,” Leah tells her. “You look like the f------ Madonna.” And still, Natalie cracks.