This book is the best introduction to TDD I've seen, and for such a tiny book, has a surprising amount of depth. The examples are in Python, but are simple enough that they'd be easily translated into any language. I've gotten new things out of it every time I've reread it, and it's one of the few tech books I have actually gone back and reread.
A classic, and with good reason. It's a great reference to have around, but it also includes some more general chapters that are well worth reading. If you like this book, I also recommend 'Refactoring to Patterns', by Joshua Kerievsky, which is a similar catalog of refactorings, but to (and equally importantly, away from) some of the design patterns in the Gang of Four book.