This book talks about the role of a tester in an XP project. So it is about acceptance testing, not unit testing (see Test-Driven Development by Kent Beck for that).
The long and short of it is that I would really like to run an XP project with people who have read, understand, and become excited about this book. My experience is that a project suffers from not having someone who’s job is 100% quality. Programmers (myself included) will sadly have too much at stake to be effective at testing their own code. Having someone who is dedicated to acceptance testing on the team will also be a good way of keeping the iteration cycle short while delivering software of good quality.
I did notice a few weird things about the book, though. The choice of webART as an example of a web acceptance testing tool seemed to me to be a bad one. My impression from the book is that webART is a rather verbose language. The book also suggest estimating down to hours (and in some cases, below that). Personally, I don’t have enough predicability in my progress to do this, but I guess your milage may vary.
All in all, a very good book. I hope to get a tester who works with the mindset the book describes on my next project.