These US Soccer Box documents should be in every referee's personal library, regardless of Referee Grade. Additional Publications (Laws of the Game Made Easy, Offside Made Easy, ...) can be obtained at the US Soccer Resource Center.
Player Development Initiatives (PDI):
The PDI describes the standards for small-sided play (4v4, 7v7, 9v9) and addresses US Soccer's Concussion Initiative, Build Out Line Practical Applications, and the modifications to the Laws of the Game for 6U through 12U.
US Soccer's Guide to Procedures for Referees, Assistant Referees & 4th Officials:
Although no longer published, the Guide to Procedures describes the standard practices for positioning, signaling, and mechanics to be followed by the referee crew. The responsibilities, tasks and duties of the Referee, ARs, and 4th Official are also included. The Guide has been replaced with the USSF Referee Program Pocket Guide.
US Soccer Referee Certification Requirements:
These documents describe the minimum certification requirements (training, testing, fitness, and evaluation) for USSF Referee Grades 9, 8, & 7, State Referee Grades 6 & 5, and Emeritus and how-to print out your USSF Registration Card. NOTE: Iowa's certification requirements for Grades 7 and 6 are different. (Check under Supplemental Publications for specifics.)
International Football Association Board (IFAB) Laws of the Game:
These are the 17 Laws of the Game from the IFAB. This 212 page PDF document also includes a Summary of Laws Changes and Practical Guidelines for Match Officials.