High Performance MySQL - by Baron Schwartz, Peter Zaitsev and Vadim Tkachenko
I've worked a lot with MySQL, over the past few years. I'm completely self-taught, however, and obviously, I've had gaps to deal with. While Google is dandy in a pinch, a book like this really fills in the blanks and provides a far more comprehensive understanding.
Granted, if you're just starting out with MySQL, this is probably not the best book for you. Just looking at it from a learner's perspective, you'd better have at least a basic grasp under your belt or this book is going to be a tough one to tackle.
If you've got a good background in it though, and are looking to be a real expert in the field, I'd definitely recommend this book. Even if you've already read the second edition, this third edition is definitely worth the purchase as the information is updated and the authors really expand previously presented topics.
My overall take: It's well written, well organized and easy to reference when needed.