But throughout that gilded childhood, Eberstadt longed for another existence, for the footloose life of the willfully dispossessed. In her novels, idealists and fast-trackers wrestled with thorny problems of love and social identity. After her family’s move to the French countryside, her lifelong fascination with Gypsies inspired a nonfiction book about their haunted music and lives. “Flamenco,” she wrote, “is the art of desperate measures, the winning of a fugitive grace from failure, bankruptcy, shame.” That fugitive grace, that rag-picking of hope from ruin, resurfaces in Eberstadt’s shrewd and sensuous fifth novel, “Rat.”