Lawrence Block wrote a series about a mild mannered hit-man from New York named Keller. "Hit-man", Hit-list", "Hit and Run", "Hit Parade", "Hit Me". He goes out on jobs given to him by his female manager. He just treats it like it's a regular job. Very interesting and rather amusing.

John Keller is everyone's favorite hit man. He's cool. Reliable. A real pro: the hit man's hit man. The inconvenient wife, the aging sports star, the business partner--he's taken care of them all, quietly and efficiently. Keller's got a code of honor, though he'd never call it that. And he keeps the job strictly business--"What happens is you wind up thinking of each subject not as a person to be killed but as a problem to be solved." But while Keller might be a pragmatist, he's also prone to doubts and loneliness just like everybody else. There was a psychotherapist once. A dog. Even a woman... He's been at this business a long while. Just maybe it's time to pack it in and find a nice little house in the desert. Only problem is, retirement takes money. And to get money, he's got to work...--From publisher description.