Los Angeles in the 1960s gave the world some of the greatest music in rock ’n’ roll history: “California Dreamin’” by the Mamas and the Papas, “Mr. Tambourine Man” by the Byrds, and “Good Vibrations” by the Beach Boys, a song that magnificently summarized the joy and beauty of the era in three and a half minutes. But there was a dark flip side to the fun fun fun of the music, a nexus between naive young musicians and the hangers-on...