The Julian Mercer series: 

Julian Mercer – his business card reads K & R specialist, but there isn’t much that the former Special Air Service commander won’t do. Ever since his forced retirement, Mercer and his team have been working extensively to negotiate the safe return of kidnapping victims, but that isn’t enough to satisfy Julian’s darker desires. His own life-altering tragedy has left him on the brink of insanity and violence, and only his team and his job can keep him from falling over the edge. But this latest missions could be a game-changer.