The Friends of Eddie Coyle — George V. Higgins

Eddie works for Jimmy Scalsi, supplying guns for bank jobs from gun-dealing Jackie. A cop named Foley is leaning on Eddie to finger Scalsi while Dillon, a full-time bartender and a part-time contract killer, is pretending to Eddie's friend.
These are the people who make up the world of unsavory characters. Looking at his "friends." Eddie knows that he has a problem -- whom to sell out to avoid being sent up the river -- or worse.