The Plunge by David Goodis

Roy Childers is a detective who has tried to stay clean in a corrupt police force. He becomes obsessed with finding escaped convict Dice Nolan. But his stakeout leads him into an illicit affair and a fierce jealousy that causes him to lose control. When the truth about a murder emerges, Childers plunges into darkness, realizing that the line between right and wrong is not always clear. A gripping psychological noir exploring the fall of a moral man.