Packt Publishing has a new book on Drools, the popular open source Rule Engine.  The book covers current functionality outside the plain rule engine (i.e. the BRMS, CEP, Drools Server, etc.)  

The book can be found at http://www.packtpub.com/drools-developers-using-jboss-cookbook/book

A review can be found at http://rickwagner.blogspot.com/2011/10/book-review-for-drools-developers.html