Old and probably hard to find, but they did make a good movie out of it.

"Report To The Commissioner." By James Mills.

In the Preface we learn that there's been a major scandal in the NYPD and two cops are dead. The writer is a local newspaper reporter who has heard bits and pieces of the story, mostly conflicting. One night he's at the bar and someone hands him a thick envelope. Inside is the internal investigation conducted at the behest of the Police Commissioner.

The novel is in the form of taped interviews, internal memoranda, a magazine profile of one of the cops, medical reports, etc.