Bibliography

Concepts and Background

Some Guidelines For Deciding Whether To Use A Rules Engine, by George Rudolph, http://herzberg.ca.sandia.gov/jess/guidelines.shtml
RETE Algorithm, by cut-the-knot http://www.cut-the-knot.org/classes/Last.shtml
RETE Algorithm on wikipedia (really good explanation!), http://en.wikipedia.org/wiki/Rete_algorithm
Application Validation Strategies, by Dietz (date unknown). Describes a framework to validate data within applications - a neglected topic in IT-literature...
Rule-Works User Guide, by RuleWorks. An excellent introduction into the inner workings of most modern rule engines, covers the (essential!!) recognize-act cycle.
Business Rules: What Are They Really, by Business-Rules-Group.
Business Rule Process Management, by Paul V. Haley.

Open source rule management: JBoss Rules and Jess deliver solid rule tools and respectable performance at a nice price, by James Owen, InfoWorld, November 2006. Compares JBoss-Rules and JESS. Good comparison.
Building Enterprise Services with Drools Rule Engine, by Birali Hakizumwami, January 2007. Published on ONJava.com (http://www.onjava.com/). Uses mortgage underwriting as example, based on JBoss-Rules 3.0.4. Good example, small and straightforward ruleset, covers conflict resolution.
Real-World Rule Engines, by Geoffrey Wiseman, June 2006. Excellent introduction, covering several architectural topics as well. Followed by an inspiring discussion.
Give Your Business Logic a Framework with Drools, by Paul Browne, August 2005. Contains a (very) small stock-trading example, easy reading.
Ruling Out: Rule Engines and Declarative Programming Come to Java, by N. Alex Rupp, August 2004. A short coverage on "why rule engines" with a small example.
Getting Started With the Java Rule Engine API (JSR 94): Toward Rule-Based Applications, by Qusay H. Mahmoud, July 2005. Covers a little of JSR 94 (which I personally dislike). Good intro.
Implement business logic with the Drools rules engine, by Ricardo Olivieri, May 2006.

Writing the Rules, by Simon Bisson, February 2007. Compares ILog JRules (V.6), Blaze Advisor (V.6.1) and JBoss-Rules (V.3.0.5).

Real-Rules
JavaRules. Quoted from their homepage:

Information related to building business rule applications or rule-based systems using the Java programming language.
FIT for rules: Rule testing framework. One path to testing your rules. I'm still unsure wether it's an effective one, though. Mayby you can send me some experience reports... You'll be included in my Who's who section...
Business Rules Group
Pathfinder Business Rules Blog

Learn Prolog Now. A cool online resource on Prolog.
W.F. Clocksin, C.S.Mellish: Programming in Prolog, 2nd Edition 1984 (!), Springer Verlag. A milestone in programming literature. I bought my copy in 1986 and learned Prolog from it...

Please note: I'm NOT affiliated by any means to any rule-engine vendor (although my PayPal account would be most happy to receive major donations).