Repairman Jack is a character in a series of novels by F. Paul Wilson. Jack initially was just one protagonist in a string of related novels, but then gained his own series known as The Repairman Jack series.
This series of novels uses realistic, thriller-like situations to tell a story with a broad supernatural theme. It is technically a spin-off branch of the aforementioned larger, overarching supernatural horror series, The Adversary Cycle.
Repairman Jack is a self-titled "fix-it" man, but not in the common workshop sense. He is something of an underground mercenary, hired by everyday people to fix situations that cannot be dealt with through legal means (e.g. by blackmail). He is careful about who he agrees to do fix-its for, preferring innocent, desperate citizens being victimized with no one else to turn to. These fix-its usually begin simply, but grow into complex problems that begin to involve more and more science-fiction and supernatural elements as the novels continue. The second novel, Legacies, is the only one that is completely free of any overtly supernatural elements.
The Repairman Jack series is the name given to fifteen (as of 2011) horror/thriller novels written by American author F. Paul Wilson, as well as several standalone short stories.
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".
The series proper begins with The Tomb and ends with Nightworld. However, these books are also part of another series of novels by Wilson known as The Adversary Cycle. The Repairman Jack series was restarted with Legacies in 1998. This newer series is following chronologically from the events of The Tomb to the events immediately preceding Nightworld.