Lifeless by Mark Billingham

Tom Thorne's career is on the skids. Having seriously crossed the line on his last case and depressed over the recent suspicious death of his father, the once ambitious police detective has been reduced to pushing papers... and is being encouraged to take a prolonged leave of duty.But someone is stalking the city's most destitute citizens. Three homeless men have recently been kicked to death, each brutalized corpse discovered with a banknote pinned to its chest. With nothing to lose, Thorne volunteers to try to find the killer—taking to the streets he knows so well from his days as beat policeman and as a homicide detective, but this time joining the squalid ranks of life's rejects. In this harsh and harrowing netherworld, with its own rules and moral codes, a shocking link between the brutal crimes and a fifteen-year-old atrocity could end up costing Thorne what little life he has left.

To find a vicious killer of three homeless men, Detective Inspector Tom Thorne must go undercover and enter a dark and disturbing world with its own rules, its own hierarchy, and plenty of secrets.