Building Parsers in Java
The OFBiz Rule Engine is based on the Logikus language in this book, but is
far from the full extent of the excellent material in this book. It explains
Steven Metsker's parser framework (which is also included in OFBiz since the
Rule Engine uses it) which is an excellent and flexible free form text parser.
The discussions of different types of languages (imperative, logical, declarative,
etc.) are alone worth the read.