Scarred (Henry Christie 28) by Nick Oldham

Henry Christie has never forgotten the day he was brutally assaulted as a young constable while chasing a teenage shoplifter down a Blackpool alley. Tommy Benemy went missing soon after his arrest, but although Henry promised Tommy’s mother he'd keep an eye on the case, her son was never found.
Now retired, Henry reluctantly agrees to join the Cold Case Unit as a civilian investigator, teaming up with the volatile DS Debbie Blackstone, who’s carrying scars of her own. When an old case leads them to a serial rapist, and a gruesome scene takes Henry back to his old promise – and failure, the pair find themselves confronting their demons as they unearth a deadly criminal conspiracy spanning decades, and chilling secrets desperate individuals will go to any lengths to keep hidden.