When Jerry Leader was growing up, he made himself a toy set of DJ equipment. Two empty cereal boxes were the turntables. For the records, he cut circles out of cardboard. The needle, a plastic spoon. He would sing to himself, "mixing" the tracks. Leader grew up in an 18-story apartment building in the Bronx, New York City, during the 1970s and 1980s, with his parents and eight siblings. The address was 1520 Sedgwick Avenue. It's a tall, unremarkable high-rise overlooking an expressway. But he says the building, and his unit, were always filled with...