From time to time a book is recommanded on the mailing-list, you can find it here with the original comments !
TDD is a lot about the mechanics of coding, but it says you are done when there is no duplication. DDD considers the code in constant churn, always renaming as the domain becomes clear and also feeding terms back into the domain as the code evolves.
Anyways, it's a great book :) [source]
Books by contributors
Two of SimpleTest contributors have written books about PHP. Both have a lot of examples with the SimpleTest Unit testing framework.
One way to help the team of contributors is to buy books from this page through Amazon (with the simpletest-21 tag). We all love reading stuff that's both interesting and challenging.