Another book on testing I'm in the middle of is Weinberg's Perfect Software: And Other Illusions About Testing (not specifically agile, of course). Interesting how books often come in related bundles. These three could be the "winning agile testing trifecta."
Gojko's excellent book is focused on using specification by example and agile acceptance testing to improve communication and ensure that the team delivers the correct business value. Gojko explains some great techniques such as specification workshops to help elicit examples and requirements. We cover similar ground in Agile Testing, but Gojko approaches it from a different angle and delves more deeply into why software projects suffer from a communication gap and how to fix it.
Our book goes into more details than Gojko's about helping testers transition to agile and helping teams address cultural and organizational challenges that can get in the way of the agile "whole team" approach. Our book has a big section about the Agile Testing Quadrants which I've mentioned in other posts here and how that can be used to accomplish all the different purposes of testing, with a team approach. We also follow a tester through "an iteration in the life" to see how testers contribute on a daily basis throughout the iteration, how they collaborate with customers and programmers.
Personally I think everyone should read both books!
I haven't gotten to Weinberg's book yet but I'm sure it's excellent. Nice to have multiple sources of information to help with testing in agile teams!
Co-author, with Janet Gregory: Agile Testing: A Practical Guide for Testers and Agile Teams (Addison-Wesley, 2009) http://lisacrispin.com