Midnight by Stephen Leather

Summary

Jack Nightingale, ex-detective turned PI, has a dilemma.  Everyone he speaks to winds up dead, and with each body comes the same chilling message, “Your sister is going to hell, Jack Nightingale”. Having never met his sister, and with no idea where to start, Jack and his assistant Jenny embark on a journey into the supernatural to save a soul.

Things take an unexpected turn when his sister turns out to be notorious for all the wrong reasons. As the body count starts piling up Jack becomes implicated in a murder himself, and begins to wonder whether his sister was framed. Determined to help her, Jack calls on forces from the dark side - Proserpine, a Devil’s aide. He has to strike a bargain, but making any sort of deal with a devil can be a very dangerous thing. He is forced to question whether one person’s soul is worth such a high price

His attempts at a double cross, to ensure his own soul isn’t damned, anger those in darkest depths of hell. Dodging devils and assassins, while dealing on this plane with the Metropolitan police (the Met), Jack looks to pull off the impossible. As he does, he learns a great deal about his heritage, proving that in some cases ignorance is bliss.