Doors of the Night by Frank Packard

Things dont look good for Billy Kane when his millionaire employer David Ellsworth is found murdered and robbed. All evidences suggest that he is the one who did it, and those circumstances dictate that he must go in hiding. As Billy wonders further in New Yorks criminal underground, he gets mistaken for a notorious Buddy Morgan, known as The Rat, one of the bosses of the underworld. Seizing on the unlikely opportunity to clear his name Billy decides to pit the underworld in a fight on the side of justice against the police.