A book about ANTLR is something that I have long lusted after. It is something that I keep planning to learn, but the learning curve is too high to do this informally. Looks like I am getting my wish, The Definitive ANTLR Reference is in beta book now.
ANTLR is a compiler compiler, which makes it a great tool to write languages (Boo's parser is written in ANTLR, for instance). And although I would generally use Boo for DSL, it is important to me to understand the parser as well.