Hit Man — Lawrence Block

Keller is your basic Urban Lonely Guy. He makes a decent wage, lives in a nice apartment. Works the crossword puzzle. Watches a little TV. Until the phone rings and he packs a suitcase, gets on a plane, flies halfway across the country... and kills somebody. It's a living. But is it a life? Keller's not sure. He goes to a shrink, but it doesn't work out the way he planned. He gets a dog, he gets a girlfriend. He gets along.

You've never met anyone like Keller.