Motor city blue - Loren D. Estleman

Marla Bernstein is a pretty, dark-haired teenager...who also happens to be the ward of Ben Morningstar, a semi-retired mobster who prefers to keep family business out of the newspapers. When Maria suddenly disappears, the gang boss is forced to call in private eye Amos Walker, who quickly learns his new employer doesn't take 'no' for an answer when he offers a job opportunity.
Unfortunately, the only clue to Marla's whereabouts is a pornographic photograph that clearly proves she's become part of a world that disgusts even her criminal guardian. The photo, in turn, leads Walker into the seedy world of Detroit's porn shops and blue movies, where Marla's trail becomes even murkier...and increasingly more dangerous to follow. As first cases go, Walker certainly could have asked for one less challenging...