The Persuader by Lee Child.
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 troubleóbig, violent, complicated troubleófinds 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 Francis Xavier Quinn got away with murder. Now a chance encounter outside Boston's Symphony Hall 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.