If you haven't checked out USSoccer's  refereeing materials, you are missing out on some great stuff!

US Soccer provides a host of instructional and informational material for referees on its website 

The Referee Week in Review is a week-by-week discussion of situations and incidents, primarily in Major League Soccer, but applicable to all levels of play. The principles on which the referee decisions should be made are always addressed along with a description and analysis of he incident, which will usually include videos.

Past WIR discussions, arranged by law, can be found here  .

You can also find recent memoranda and policy papers on specific issues. For instance, there is a complete description of the procedures for Kicks From the Penalty Mark to decide advancement or tournament winners. Another recent paper discusses the referee's duties and procedures for Head Injuries.

Under Referee Development   you can find more position papers, download the Referee Administrative Handbook and instructional materials, and view the FAQ for new referees.

Be sure to check out the blog, featuring members of the National Referee Program Staff and current and former FIFA Referees from the US.

Under the Laws of the Game heading  you can download copies of the LOTG, of course, including the Interpretations and Guidelines (which have replaced the Additional Instructions, the Questions and Answers, and many of the International Board Decisions of past years. Also here, you can find position papers and guidance documents on many issues.