The Repairman Jack series is the name given to sixteen (as of 2012) horror/thriller novels written by American author F. Paul Wilson, as well as several standalone short stories and 2 spin-off trilogies.
The series revolves around the adventures of a Manhattan-based "urban mercenary" who calls himself Repairman Jack.
Jack is a man with no last name or legal status who makes his living "fixing" situations for people who cannot find help elsewhere.
His work has not only led him into violent encounters with the criminal underworld, but also into the midst of a behind-the-scenes cosmic battle for Earth between two supernatural forces: one known as the Ally, which is benign towards humanity, and one as the Otherness, which is hostile towards it and generally considered "evil".
However, those two books are also part of another series of novels by Wilson known as The Adversary Cycle.