Book: Test-Driven Development in Microsoft.NET
At last UGIdotNet workshop (22th, June) I received this book.
Yesterday I started reading it, and it was so cool that I've already finished it, from cover to cover.
The book is really well done, it explains TDD without being too abstract. In fact, every step is explained with a practical example.
If you need a book that explains not only the theory, but also how to use NUnit to obtain your results, this is the right one.