Drools 5 is available for download. The corresponding svn tag is http://anonsvn.labs.jboss.com/labs/jbossrules/tags/18.104.22.168597.FINAL/. You can read the announcement on the Drools blog.
FWIW, there’s also a book available released by Pack Publishing.
It covers basics, like getting the software, setting up the Eclipse IDE and making your first steps with Guvnor (a web application for authoring and managing rules) as well as more advanced concepts like support for a domain specific language, tools for rules testing and the internals of the Drools rule engine.
You can read more about this book on their website and chapter 4, Guided Rules with the Guvnor is available for free. It’s a quick introduction to start with Guvnor. It begins with building a data model in Eclipse, goes through building a business rule with the guided editor and ends with running a test scenario.