There are a number of outstanding referee websites that provide instruction and video clips of actual game situations that can be helpful to referees:
Ask a Soccer Referee
This is an excellent website where referees from around the country can submit questions to USSF and get official answers (not just some "expert" referee's opinion -- "expert" has been defined as a has-been drip under pressure!). It's interesting to read the question submitted and then to try to answer it yourself before reading the official answer. It's a challenge! Sometimes those answering the questions rag on some of the "inventive referees" who come up with all sorts of weird law interpretations. The link to the site is http://www.askasoccerreferee.com/
Law Book and Guide to Signals
You can download a .pdf file of the laws from the following website: http://www.ussoccer.com/Referees/Laws-of-the-Game.aspx
You can also download a .pdf file of the "Guide to Proceedures for Referees, Assistant Referees and Fourth Officials" at the same website as above. This contains all the approved signals and procedures for referees (sometimes referred to as "mechanics", that is, how to signal, where to be positioned, etc.).
In-Depth Explanations of the Laws
If you need help understanding a particular law, USSF publishes "Advice to Referees", which provides great explanations of all facets of each law. It is organized by each law, so it's easy to find answers about offside issues by going to section 11 of Advice to Referees, which corresponds to the offside law, which is Law number 11 in the Law Book. The link to download the .pdf file is http://www.ussoccer.com/Referees/Referee-Development/Instructional-Materials.aspx