Midnight. Stephen Leather

Jack Nightingale found it hard enough to save lives when he was a cop. Now he needs to save a soul - his sister's. But to save her he has to find her and they've been separated since birth. When everyone Jack talks to about his sister dies horribly, he realises that someone, or something, is determined to keep them apart. If he's going to save his sister, he's going to have to do what he does best - negotiate. But any negotiation with the forces of darkness comes at a terrible price. And first Jack must ask himself the question: is every soul worth saving?