26-year-old Brit Bennett’s sparkling debut novel, The Mothers, came of age over eight years and several drafts. She began penning the tale of youthful indiscretions and betrayals while just a teen. Then she carried it with her through college at Stanford and to MFA and postgraduate fellowship programs at the University of Michigan, where she torched and remade the story repeatedly. The pull of the characters and drama at Upper Room Chapel, a black church in a California beach town, kept her . . .