The Midnight Line by Lee Child

Reacher takes a stroll through a small Wisconsin town and sees a class ring in a pawn shop window: West Point 2005. A tough year to graduate: Iraq, then Afghanistan. The ring is tiny, for a woman, and it has her initials engraved on the inside. Reacher wonders what unlucky circumstance made her give up something she earned over four hard years. He decides to find out. And find the woman. And return her ring. Why not?

So begins a harrowing journey that takes Reacher through the upper Midwest, from a lowlife bar on the sad side of small town to a dirt-blown crossroads in the middle of nowhere, encountering bikers, cops, crooks, muscle, and a missing persons PI who wears a suit and a tie in the Wyoming wilderness.

The deeper Reacher digs, and the more he learns, the more dangerous the terrain becomes. Turns out the ring was just a small link in a far darker chain. Powerful forces are guarding a vast criminal enterprise. Some lines should never be crossed. But then, neither should Reacher.