Rough Justice - Stephen Leather

The seventh book in the bestselling Dan 'Spider' Shepherd series. Villains across London are being beaten and killed by vigilante cops. Crime rates are falling, but the powers that be want Dan 'Spider' Shepherd to bring the wave of rough justice to an end.
Shepherd is never comfortable investigating cops, but working for the Serious Organised Crime Agency means he has no choice. He goes undercover with an elite group of officers at the sharp end of policing, risking their lives daily on the worst streets in the capital. But then his family moves into the firing line . . .