Play Dead by Bill James

Following a deep investigation into an incident in another Force, Colin Harpur and his boss, Assistant Chief Constable Desmond Iles, uncovered an unpalatable truth: there is corruption in the ranks.
Good detective work allowed the pair to arrest and charge a fellow policeman, who murdered an undercover officer in an attempt to keep a crook―police racket secret. But their work is not yet done. Who ordered the execution? Why? Just how high did the conspiracy go? And ― and most importantly ― does the crooked power group behind the assassin still flourish?
There are questions that remain to be answered ― so when the home office decides to reopen the case, it is Harpur and Iles to whom the investigation is once more handed.