This is Quality Assurance testing that specifically is geared for EXtremem Programming where programmers pair up to work and glean a lot of information from each other.
Lisa Crispin is a contributor and is also on a yahoo group list about QA for Agile projects. We are not using XP methods with my team currently but I feel this has enough information about estimation, and agile philosophies that it's worth checking out, it's also very reasonably priced.
This book remains relevant 12 years after it was published. It isn't just for XP teams either; much of what is included will benefit teams using any flavour of agile software development.