Testing Extreme Programming by LisaCrispin and TipHouse. ISBN 0321113551
See http://www.xptester.org for some more info on the book from LisaCrispin.
I liked this book because it did a really good job of answering the questions: What is the role of QA in XP? and How the heck do I handle AcceptanceTests? They propose the answer of introducing the XpTester? role, and go to great depths exploring how this role fits into KentBeck's original XP.
I also liked how they kept the tone of the better XP books such as ExtremeProgrammingExplainedEmbraceChange and ExtremeProgrammingInstalled. Chapters 19 and 20 are a great example of being extreme without being obnoxious.