Blue Moon by Lee Child

It's a random universe, but once in a blue moon things turn out just right. In a nameless city, two rival criminal gangs are competing for control. But they hadn’t counted on Jack Reacher arriving on their patch. Reacher is trained to notice things. He’s on a Greyhound bus, watching an elderly man sleeping in his seat, with a fat envelope of cash hanging out of his pocket. Another passenger is watching too ... hoping to get rich quick. As the mugger makes his move, Reacher steps in. The old man is grateful, yet he turns down Reacher’s offer to help him home. He’s vulnerable, scared, and clearly in big, big trouble. What hold could the gangs have on the old guy? Will Reacher be in time to stop bad things happening?