Kelefa Sanneh’s new book explores rock, r&b, country, punk, hip-hop, dance, and pop, not only as art forms, but as identities that emerged from countercultural movements to become the fabric of popular music. — This book felt like a mind-meld from one music nerd to another. Although at first I disagreed with Sanneh’s emphasis on the importance of genre, I came to appreciate his framework for investigating how music cultures form, rise, and are ultimately defined in retrospect.