A Brother's Keeper by Duncan Wallace

In his long career, DCI Jacob Grimm Reacher thought he’d seen it all, but in his last assignment before retirement, he’ll be faced with inscrutable and deadly mysteries:

A killer whose DNA evidence points to a soldier stationed at an army camp hundreds of miles away.

A ghost who everyone knows, but no one can identify.

A small town with a dark secret.

The “Grim Reacher” always gets his man, but in his final case, death may be seeking him out.