Red Harvest

Red Harvest by Dashiell Hammett is a gritty, hard-boiled detective novel featuring the unnamed Continental Op, a private investigator sent to the corrupt town of Personville (nicknamed "Poisonville").
The Op arrives to help a newspaper editor clean up the town but soon finds the editor murdered. Determined to uncover the truth, he sets off a brutal gang war among rival criminal factions, manipulating them against each other in an effort to purge the city of corruption. The novel is fast-paced, violent, and morally ambiguous, portraying a bleak view of human nature and institutional decay.
It's considered a classic of noir fiction and a major influence on later crime literature and film.