The Complaints by Ian Rankin

After wrapping up the John Rebus series, Rankin moves on to a new group of Scottish police -- the Complaints or our internal affairs.  Malcolm Fox has just completed a major investigation leading to the arrest of a popular detective, when he is asked to step in on a sex crimes investigation.  Problem is, he starts to like the guy, and to wonder if he could be innocent.  Fox and Jamie Breck compare notes on some interlinked cases and begin to feel that they are being manipulated behind the scenes.

A complexly plotted police procedural dealing with official and financial corruption keeps you coming back even though the characters are a bit flat.