Tip House has a website on extreme programming:

This site is dedicated to all those involved in testing on XP projects, particularly acceptance and functional testing. [xptester.org home page]

This practical tutorial for software builders demonstrates how testing is central to the extreme programming (XP) approach and explains what testing should be done and when and how it should be performed. It overviews the XP methodology, defines the roles of XP team members, shows how to write effective tests before coding begins, and sheds light on refactoring and how it relates to testing. A "road hazard survival kit" offers advice on challenges in testability, project tune-ups, large projects, and extreme testing without extreme programming. [Amazon.com: Books: Testing Extreme Programming]