Rich Cohen has established himself as the poet laureate of what his first book dubbed “Tough Jews” — the gangsters of that volume, the anti-Nazi guerillas of The Avengers, and now the record company executive Leonard Chess of his splendid new work, Machers and Rockers. A native of the Chicago area, Cohen writes about one of the city’s greatest contributions to American music, Chess Records, which recorded such blues masters as Muddy Waters and Howlin’ Wolf as well as such early rock-nrollers as Chuck Berry. Leonard Chess was no musicologist; he was a rough-hewn Polish immigrant who saw a way to make a buck with the music most whites wouldn’t touch. Yet, in Cohen’s telling, he did reach a kind of empathy with and respect for the musicians, most of them relative newcomers to the urban North from the sharecropping South of Jim Crow. Too many books about American popular music treat its entrepreneurs (later to be millionaires and moguls) either as creative geniuses blessedly above the hurlyburly of commerce or else as corrupt con men pillaging African-American art for white gain. Cohen presents a far more subtle and supple picture of mutual support and mutual exploitation on the part of Chess and his leading musicians; the blacks and whites of Machers and Rockers exist in brilliantly drawn shades of gray.