Star Trek: The Original Series
Episode: TOS 049 - Piece of the Action, A

Planet: Sigma Iotia II

Highly intelligent and imitative humanoid species who model their social practices after a single Federation book that was left in the Iotian's possession before Starfleet's Prime Directive came to be. The resulting societal changes reflected the book from which they had come, "Chicago Mobs of the Twenties."