Down Here — Andrew H. Vachss

For many years, Burke has carried a torch for Wolfe, a former sex crimes prosecutor who was fired for refusing to "go along to get along." They walk different sides of the street when it comes to "justice," but they share a limitless hatred of predators. So when Burke hears that Wolfe has been arrested for attempted murder, he knows something is double-wrong and deals himself in.
Burke learns that Wolfe's alleged victim is a brutal serial rapist she had personally prosecuted. He's back on the street because his conviction was reversed, and any of his long list of victims has plenty of motive to kill him. The deeper Burke gets into the investigation, the more holes he finds in the case against Wolfe. Yet the DA's office continues to press forward, and Burke has to find out what their game is. No stranger to devil's bargains, Burke reopens the rape investigation -- his own way.