This is book 4 of the "Jack Reacher" series.

This note contains spoilers for the book.

Okay, he's out of the military now.

He gets framed and blackmailed by the FBI to serve as their liaison to the military in investigating a serial killer with obvious military connections.

Yeah, the framed and blackmailed thing is a bit unbelievable. Also the killer works by hypnosis, programming people so well that she can kill them by making them swallow their own tounge in a bathtub of Army camouflage base paint. Specifically, she turns out to be the FBI's top profiler and working on the case.

Still, Reacher as an investigator still makes for interesting reading, and his interactions with his girlfriend, both of them needing a partner with fewer commitments than the other, is reasonably sane and even adult.