Peter and Rebecca Harris: mid-forties denizens of Manhattan’s SoHo, nearing the apogee of committed careers in the arts—he a dealer, she an editor. With a spacious loft, a college-age daughter in Boston, and lively friends, they are admirable, enviable contemporary urbanites with every reason, it seems, to be happy. Then Rebecca’s much younger look-alike brother, Ethan (known in the family as Mizzy, “the mistake”), shows up for a visit. A beautiful, beguiling twenty-three-year-old with a history of drug problems, Mizzy is wayward, at loose ends, looking for direction. And in his presence, Peter finds himself questioning his artists, their work, his career—the entire world he has so carefully constructed. Like his legendary, Pulitzer Prize–winning novel, The Hours, Michael...