ANTLR bookEdit
There is some documentation on the official site but the best documentation by far is The Definitive ANTLR Reference by Terence Parr, the author of ANTLR:
ANTLR itself is free software released under the BSD license so purchasing the book is a great way to support Terence’s efforts. I purchased the "beta" version of the book in PDF ($25), which includes access to the final version of the book when it is published (in May 2007). The quality of the Pragmatic PDFs is very high.
You can also support by posting a review of the book at Amazon, as your review may encourage others to make a purchase: