Laws of the Game
You'll find links to official sources of information about the Laws of Soccer below. Most of these files are in pdf format, are located on external websites and take a while to download.
USSF Advice to Referees (a very useful document by the US governing soccer body on how to interpret FIFA laws))
USSF Guide to Procedures (official descriptions of referee mechanics and signals)
AYSO National Rules and Regulations (AYSO specific laws)