Diego Adrian Naya Lazo has just published an OSWorkflowBook "OSWorkflow: A guide for Java developers and architects to integrating open-source Business Process Management".
I've no idea if the book is any good or not but it has a whole chapter on Drools integration:
Chapter 5 introduces and integrates Rules engine and Drools open source rule engine.
So if anyone reads this book, please do post us a review we can place here. And Diego, any chance you could email me chapter 5, as a free sample :)
What I found more interesting is that a non JBoss workflow engine put in a chapter about Drools integration, but the JBoss jBPM book "Business Process Management with JBoss jBPM" didn't. I did notice that the book refers to us as just Drools, I hope somewhere prominently in the chapter that JBoss are given credit.