Undercover - Bill James

After a gang shooting involving an undercover police officer, Colin Harpur and his boss Assistant Chief Constable Desmond Iles are called to another Force's ground to investigate what the Home Office see as spectacular failings. Harpur can imagine the pressure the officer would have been under. If a gang decided to kill, a spy would have to go along with it. But with careers of fellow officers - who might be in secret, dangerous alliance with villains - at risk, Harpur knows that he and Iles have an exceptionally tough inquiry ahead.