"Twenty years ago, Abigail Sorenson’s brother Robert went missing one day before her sixteenth birthday, never to be seen again. That same year, she began receiving scattered chapters in the mail of a self-help manual, the Guidebook, whose anonymous author promised to make her life soar to heights beyond her wildest dreams. The Guidebook’s missives have remained a constant in Abi’s life—a befuddling, yet oddly comforting voice through her...