A memoir by the teen idol and pop star reveals the real story behind his phenomenal '70s stardom—and the sadness that shadowed it.Barely out of his teens, David Cassidy landed a role on a new sitcom about a musical family that toured in a psychedelic bus. The critics blasted it—but TV viewers loved it! And the young female audience especially loved Keith Partridge. Not only did they tune in each week, they bought The Partridge Family's hit single, "I Think I Love Y...