The Midnight Line by Lee Child

Reacher steps off a bus to stretch his legs.  There’s not much to see.  The only thing open in a pawn shop.  He finds a ring there, a West Point graduation ring, size small, like a woman’s ring.  It has an inscription.  He knows what hell a woman had to have gone through to earn that ring, so he decides to buy it and track her down to return it.  He gets the name of the person who sold to the pawn shop and where they’re from.  He goes there.

Reacher finds a network hidden in the back of a self-serve laundry that nobody uses.  As he tracks the origin he finds a Major, like his own rank, who did five tours in Afghanistan and Iraq.  She was injured by an IED and like many soldiers returning from war, the meds ran out, so she’s gone back to her childhood home in the mountains of Wyoming and is surviving there.

Her twin sister has also been looking for her, so when Reacher helps track her down, there is a lot to discuss and do, Reacher’s way.