"John Lennon was one of the world's most influential people. Mark David Chapman was one of the most invisible. By the end of 1980, the Beatles had been broken up for a decade -- a decade John Lennon had spent in search of his true identity: singer, songwriter, activist, burn out. "It's the perfect time to be coming back," he declared. Except that Lennon was a marked man. As early as the Beatles' controversial 1966 American tour, the band had feared...