The titular organization is basically the mob or Mafia but it doesn't go by that name. The Outfit wants Parker dead for past deeds they think were wrong but Parker knows were right. The guy they send to do the job louses it up and now it's Parker who's hunting the Outfit. In fact through a letter-writing campaign he encourages his fellow professionals - not friends, not for Parker, but men he's worked with - to knock over various illicit "businesses" run by the Outfit. This works extremely well, like clockwork even, and soon the Outfit's out nearly a million and the head, Bronson, wants Parker... you guessed it, dead. But Parker's got other plans.
My mind is boggled by the effortless way in which Westlake depicts the criminal underworld and their Byzantine cons and double-crosses and set-ups. One great thing about The Outfit is the time Stark spends detailing said cons, like numbers-running or betting on horses. Even though I love good crime fiction I find it difficult to grasp these logistics. He makes it easy. Stark's spare and precise prose makes you feel smart and tough at once. I like that in a book. Hell, I love that in a book.