Savages — Bill Pronzini

The police said it was an accident, the dead woman's sister said it was murder... and that she knew who did it. Nameless isn't certain, but the more he learns about Nancy Mathias's life, the more inclined he is to accept the likelihood of murder--especially as the players still alive become more and more distasteful. Combine that with the situation Jake Runyon, one of the agency's partners, is facing as he searches for a young man who is either a murderer or a victim, and life at their San Francisco detective agency has everyone on edge.