The calling / Neil Cross.

Enhanced descriptions from Syndetics:

Is Luther a force for good or a man hell-bent on self-destruction?

Meet Detective Chief Inspector John Luther. He's a homicide detective with an extraordinary case-clearance rate. He's obsessive, instinctive, and intense. Nobody who ever stood at his side has a bad word to say about him. And yet there are rumors that Luther is bad--not corrupt, but tormented. After years of chasing the most depraved criminals in London's gritty underworld, he seethes with a hidden fury he can barely control, making him do things any other detective wouldn't and shouldn't do.

Luther: The Calling is the story of the serial killer case that tore Luther's personal and professional relationships apart and propelled him over the precipice--beyond fury, beyond vengeance, all the way to the other side of the law.

Meet Detective Chief Inspector John Luther. He's a murder detective with an extraordinary case clearance rate. He's obsessive, instinctive, and intense. Nobody who ever stood at his side has a bad word to say about him. And yet there are rumors that Luther is bad--not corrupt, not on the take, but tormented. He seethes with a hidden fury that at times he can barely control. Sometimes it sends him to the brink of madness, making him do things he shouldn't--things well beyond the limits of the law.
Edgar Award-winning writer Neil Cross has created one of the most compelling characters in modern crime fiction--a man who may be a force for good, or hell-bent on self-destruction. For fans of the award-winning series starring Emmy-nominee Idris Elba, and for all lovers of crime fiction, Luther is a compelling, compulsively readable novel by the writer hailed by The Guardian as "Britain's own Stephen King."