Persuader by Lee Child

Jack Reacher…The ultimate loner…
An elite ex-military cop who left the service years ago, he's moved from place to place…without family…without possessions…without commitments.
And without fear. Which is good, because trouble-big, violent, complicated trouble-finds Reacher wherever he goes. And when trouble finds him, Reacher does not quit. Not once. Not ever.
But some unfinished business has now found Reacher. And Reacher is a man who hates unfinished business.
Ten years ago, a key investigation went sour and someone got away with murder. But a chance encounter brings it all back. Now Reacher sees his one last shot. Some would call it vengeance. Some would call it redemption. Reacher would call it…justice.