The Good Guy by Dean Koontz

The Good Guy is a thriller novel by American author Dean Koontz, which was released on May 29, 2007.
Timothy Carrier is an unassuming stonemason who, while having a beer at his regular bar, is accidentally mistaken for a hitman by a stranger who hands him an envelope containing $10,000 and a photo of the intended victim, a writer named Linda Paquette. The real killer arrives soon afterwards, and Tim manages to bluff him by pretending to be the client, saying he's had second thoughts, and cancelling the hit while giving the killer the $10,000 as a "no-kill fee". Tim covertly follows the killer outside, and is shocked when the killer places a roof-mounted police light on top of his car before driving off; the hitman is a cop. Tim tracks down Linda and flees, using his network of friends to stay one step ahead of harm, while being tracked by the hitman. Over time, Tim and Linda develop feelings for one another. Linda confesses to Tim about her haunting childhood; her parents, former preschool teachers, were unjustly accused of committing horrific sexual crimes on their students, including their daughter. Linda was separated from her parents, as both of them had to go to jail. Eventually, Tim takes sanctuary in his mother's house, and confronts the hitman which ends with a gunfight. Tim kills the hitman, just as a mob of FBI agents raid into the house. To calm down the waters, an FBI agent reveals the hitman had actually been employed by an American politician. Long ago, an aide of that politician had lunch with a member of a terrorist organization... (Summary by Wikipedia)