The Developer's Guide is the second book about Drools available from Packt. It nicely extends the Drools documentation showing a new point of view at almost all features.
Although it wasn't always easy to follow I definitely recommend this book to developers who either are starting their journey with this rule engine or are already familiar with Drools. Examples are not trivial 'hello world' applications. Especially my favorite Drools Fusion chapter about complex event processing is built up on quite advanced rules for a fraud detection system.
One chapter is dedicated to performance, which is a must-read for all who have to deal with a huge amount of rules and facts. It explains how to write rules to take advantage of node sharing and how to speed up fact propagation using parallel mode.
Another chapter is dedicated to a complete 3-tier JEE application. It's built up on Hibernate (for persistence), Bitronix transaction manager (for transactions), Spring MVC (for presentation) and obviously Drools (for business logic).
What's also worth to note is the test driven development style presented in almost all examples.
To sum it up, building rule based services is not an easy task. This book introduces the Drools rule engine and deals with almost all its capabilities - starting with the core engine functionality, going through decision tables, complex event processing and business process presentation ending with advanced topics covering performance and integration points with other systems.