Sanaë Lemoine’s début novel, “The Margot Affair,” is about a seventeen-year-old high-school student whose father, a high-ranking official, does not acknowledge her or her mother publicly. In telling Margot’s story, Lemoine drew upon her own complex family history: when she was twenty-one, she discovered that her father had a secret second family. In an act of literary justice, Margot decides to take action to force her father’s public acknowledgement, in a way that Lemoine herself did not. Plus, Adam Gopnik explores the predicament of an aging population. People of retirement age will outnumber children in the U.S. in about fifteen years, but they are poorly served by the field of design. Gopnik sets out to experience their difficulties firsthand.