Two childhood friends grow up and apart, marry and come to crisis. That’s the rough arc of NW, a four-part novel set in housing projects in London’s northwest district, where social mobility is (at least in this story) both a goal and a trap. Zadie Smith, whose last novel was the amiable E.M. Forster tribute On Beauty, plays a more dangerous game here: with its disparate sections and at times elliptical characterization, NW risks alienating readers accustomed to the author’s fluid, humorous prose. But Smith, who has built a solid reputation as a critic, wins points for tackling the big topics — race and class, friendship and romance, childhood and parenthood — in finely honed scenes that feel honest and human, even when humanity seems in short supply.