It's all about the scratch in Groove Music, award-winning music historian Mark Katz's groundbreaking book about the figure that defined hip-hop: the DJ.

Today hip-hop is a global phenomenon, and the sight and sound of DJs mixing and scratching is familiar in every corner of the world. But hip-hop was born in the streets of New York in the 1970s when a handful of teenagers started experimenting with spinning vinyl records on turntables in new ways. Although rapping has become the face of hip-hop, for nearly 40 years the DJ has proven the backbone of the culture. In Groove...