After using ANTLR for a class long ago and being so impressed with it, I just returned to ANTLR today. I was shocked at the lack of documentation on the web site.
I eventually typed "antlr reference" into google and found the following PDF:
It's outdated and had many no-longer-supported constructs, but paired with the changes from 2.x to 3.0 it was adequate for what I needed to do.
I can see nothing comparable linked from the ANTLR homepage. It seems like an obvious attempt to get you to buy the book. Oh well.