Red Harvest

This story first came out as The Cleansing of Poisonville.

When the last honest citizen of Poisonville was murdered, the Continental Op stayed on to punish the guilty—even if that meant taking on an entire town.
The Op was in Personville, its derogatory nickname aside, as the result of a letter to the Continental Detective Agency in San Francisco from Donald Willsson, publisher of the local paper, asking for an agent to visit. No other information. As soon as the OP arrives, the body count begins and it starts with his client!

Red Harvest is more than a superb crime novel; it is a classic exploration of corruption and violence in America and one of the greats of 20th century literature.