Essential Documentation:
-- http://www.efx-tek.com/downloads/prop-sx_docs.pdf
I'm working on a book called "Practical SX/B" for Parallax. You can download the introductory chapters here:
-- http://forums.parallax.com/forums/attach.aspx?a=15477
Note that this is in draft form and there may be small errors. The point is that it will give you a good introduction to SX/B, the langauge we use to program our accessory boards (AP-8, DC-16, FC-4, RC-4).
If you're a C programmer, there is a free C compiler (C4SX) that may work for you:
-- http://www.rwsenser.com
We haven't tried this yet, but will shortly.