The Eighth Circle

the long version

From Goodreads: "Murray Kirk runs his private investigation agency like the business it is: he isn't interested in justice or crusades, just the profit and loss account. When he's asked to act for a young policeman accused of bribery, because he knows something about police corruption in New York City, he isn't too keen. He just can't see the profit - until he meets the man's fiancee. And then Kirk's motives become uncomfortably confused, and he finds himself descending swiftly into a grey world of bookmakers, gangsters, grafters and corrupt politicians, a world where setting up an honest cop is all in a day's work."