All the world knows the end of John F. Kennedy Jr. and Carolyn Bessette's story. The hidden chapter was the gradual unraveling of their marriage as it slid from wedding-day hysterics into a spiral of depression, drugs, and violence. In an excerpt from his new book, EDWARD KLEIN explores the conflicts—over starting a family, George magazine, and life in the spotlight—that drove Kennedy and Bessette apart, despite interventions by a couples therapist and, fatally, Carolyn's sister Lauren