Glenn Frankel's Shooting Midnight Cowboy tells the story of a modern classic that, by all accounts, should never have become one in the first place. The film's boundary-pushing subject matter-homosexuality, prostitution, sexual assault-earned it an X rating when it first appeared in cinemas in 1969. For Midnight Cowboy, Schlesinger-who had never made a film in the United States-enlisted Jerome Hellman, a producer coming off his own recent flop and...