It was August 29, 1974, midday, and John Lennon, nearly 34 at the time, was up on the roof of his rented East 52nd Street penthouse. He had been at the Record Plant all week, mixing Walls and Bridges, his fifth solo album, and generally struggling to correct course after his first commercial failure (1972’s collection of mostly protest songs, Some Time in New York City). He was also recovering from his year and a half of carousing, most notoriously in Hollywood, while estranged from his…