MooTools 1.2 Beginner's Guide is an outstanding book. It aims to provide a solid foundation for MooTools new comers, and assumes a limited knowledge of Javascript, HTML, and CSS. It is not just a laundry list of MooTool function definitions, but at provides a complete coverage of how MooTools geared towards a beginner. I highly recommend it.
This is a superb book and excellent resource not just to learn MooTools, but the authors also succeed in installing some pretty important general programming concepts. The authors far exceeded my expectations for this title, and although I have been using MooTools fairly regularly for almost two years, I too picked up a few things.
This is truly a great foundational type of book, not just for MooTools but for web development in general. This book, I believe, has done something I feel is very much needed for MooTools to really take off even more than it has - it has made the framework very approachable and attractive to people not just trying to decide what JS framework to use, but also to those people who aren't even sure what a framework is.
It's fun and easy to read, chock full of code samples that are thoroughly explained and screen shots. I went through every example, and found only one occurrence of a minor error in the code provided - most books are ripe with coding errors, but this book really has none. The code is beautifully structured and everything is clearly explained and laid out.
If you're remotely interested in MooTools, Javascript, or using a Javascript framework - get this book. It is one of the best beginner type of books I have had the pleasure to come across.