There are a variety of Refereeing Resources available to Referees who want to improve their game. Some of the most valuable and up-to-date information about being a Referee in the US can be found here:
FIFA website: FIFA Refereeing Pages and the Laws of the Game (pdf version)
US Soccer website: USSF Refereeing Pages and the USSF Refereeing Resource Page
Good Luck to all Referees!