FANS OF JONATHAN LETHEM are right to be wary of his newest, Chronic City. After two inarguably great novels, Fortress of Solitude and Motherless Brooklyn, Lethem turned in 2007's frankly embarrassing You Don't Love Me Yet, a midlife crisis in book form. In You Don't Love Me Yet, Lethem abandoned his home court, the New York City about which he writes so well, to satirize the contemporary art scene in Los Angeles. Coming from one of their own, writing about a decadent Western outpost, the tinny novel may well have served to confirm the dearly held belief of many New Yorkers that LA is hollow, vapid, somehow fundamentally less than real.

It's interesting, then, that it's to New York Lethem returns to examine the notion of constructed realities head-on. Lethem builds his own Manhattan in Chronic City—the island is shrouded in fog, its weather turning to snow in the summer, its streets ravaged by a marauding tiger. Most ominously, the world is at war, the details of which are left vague. We know only that the New York Times has begun releasing a "war-free edition," and that above the earth, a spaceship and its crew are stuck in orbit, trapped by a Chinese minefield.

One of the space-bound astronauts just happens to be the fiancée of Chronic City's protagonist, Chase Insteadman. Chase is a former child actor grown into a daffily genteel playboy, residuals from his boyhood sitcom ensuring a frictionless existence. His only communication with his astronaut fiancée comes in the form of her long, pensive letters, in which she clings to the details of their relationship even as her own survival seems ever more unlikely. The letters are published in the Times, and the entire city follows the story with fascination, making Chase a sort of born-again celebrity, now as well known for his astral love as for his childhood fame.

When Chase meets a man named Perkus Tooth, he's slowly but irrevocably exposed to a world completely unlike his own—to the possibility, in fact, of other worlds existing. Perkus lives a life of unfettered geekdom. He's a film nerd who smokes too much weed, with the time on his hands to pursue every association that flashes through his drug-jarred mind. Soon Chase is joining Perkus on his pop culture rambles, listening as Perkus unfurls epiphanies about the nature of reality and how it relates to The Muppet Show, or weaves theories about Marlon Brando as the "living avatar of the unexpressed, a human enunciation of the remaining hopes for our murdered era." As Chase becomes ever more involved with Perkus' life and the characters that populate it—including a sell-out housing activist who now works for the mayor, and a prickly ghostwriter who specializes in celebrity memoirs—he soon finds himself subject to the currents and conspiracies of Perkus' paranoid yet persuasive worldview. "It was his friendship I required to usher me into the strange next phase of my being," Chase says. "To unmoor me from the curious eddy into which I'd drifted." They smoke a lot of pot together, too—and develop a particular obsession with an item known as a "chaldron," a piece of virtual pottery that exists only in the computer game Yet Another World.

As the game's title dryly suggests, Chronic City is an exercise in world building. It's also far more than a mere exercise—Lethem has an undeniable knack for tempering the asepticism of his ideas with the humanity of his characters. In its big-hearted ambition, Chronic City stands among Lethem's best.