This is the complete reference for MooTools with in-depth details and examples on every method and class in the library. PDF »PaperBack »
The online MooTorial gives a brief overview of every class and method in MooTools with examples you can run in your browser. While not nearly as complete as the book, the online tutorial is a great place to start.
Clientcide is the companion to the MooTorial, offering the largest single collection of
MooTools plugins on the net and
a blog covering client-side technology in general.
A four part, beginner focused set of tutorials that offer basic examples to follow. Heavy on examples you can tweak and play with, but a little light on the explanations.
A complete walk through of MooTools 1.2 (that you can download if you like). This walkthrough is a little more clearly aimed at teaching new-comers how to use the framework in a step-by-step introduction than the Mootorial is. A good companion if you're new to frameworks or MooTools.
This article describes the usage of this great new feature, as well as why it was developed, and how it can be used to keep your applications organized and efficient.
MooTools 1.11 Tutorials
These are still useful, though be aware that they include examples that are outdated.
This is a nice collection of "right way" / "wrong way" comparisons. Unfortunately it hasn't been updated for MooTools 1.2 (Element.setProperty is now just Element.set, for example), but it's still informative. See also the second part.(MooTools 1.11)
A nine part series covering animation, ajax, classes, and more. This is a little old now and was created for MooTools 1.11 so some of it is no longer applicable.