I would also recommend the book "Specification By Example" by Gojko Adzik. It's published by Manning and is available most places books are sold. Gojko's also set up a web site with lots of additional information at http://specificationbyexample.com/
I'm hoping that Marco's book will be another good resource.
SCJP, SCJD, SCEA 5 "Any sufficiently analyzed magic is indistinguishable from science!" Agatha Heterodyne (Girl Genius)