A while ago on the Spider forum, I asked if anyone had some good reference material that would be good for interpretting all of the technical discussion. Quackaphobe and Johndefur were kind enough to provide me a list of references.

I also found a used textbook for a course that Athabasca University offers called Physical Geology by Plummer. I bought it for $0.33 plus $6 shipping on Amazon, so it was well worth looking for.

The textbook is very easy to read, and within the first 4 chapters, I have read enough to understand what they are talking about with sills, dikes, ultramafic, felsic, etc. Really interesting reading, if you have ever looked at a rock and wondered how it was made.