A Game of Ghosts — John Connolly

It is deep winter and the darkness is unending. A private detective named Jaycob Eklund has vanished and Charlie Parker is assigned to track him down. Parker's employer, Edgar Ross, an agent of the Federal Bureau of Investigation, has his own reasons for wanting Eklund found. Eklund is no ordinary investigator -- he is obsessively tracking a series of homicides and disappearances, each linked to reports of hauntings. Now Parker is drawn into Eklund's world: a realm in which the monstrous Mother rules a crumbling criminal empire, in which men strike bargains with angels, and in which the innocent and guilty alike are pawns in a game of ghosts...