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Because of the large size of its community, MooTools has a fair share of developer resources. Here are some of the recommended resources that you should explore:
The Official MooTools Documentation (http://mootools.net/docs/). This one needs no further introduction. Any serious MooTools developer should have the official documentation site bookmarked.
The MooTools Source Repository (http://github.com/mootools/). A lot of the material in this book wasn't discovered by reading the docs but by reading and examining the MooTools source code. I therefore recommend that you spend some time working with the MooTools source code itself, which is available from the official repositories.
The MooTools Forge (http://mootools.net/forge). The Forge is the official MooTools community plugin and extension resource site. You'll see applications of the information from this book in some of the plugins and extensions published on the Forge, and I advise you to peruse the Forge and examine the published extensions as an exercise in understanding how they use the MooTools API.
The MooTorial (http://mootorial.com) and MooTools 1.3 (http://ryanflorence.com/issue-004/). These two online resources are geared towards MooTools beginners. The first is a little outdated (uses MooTools 1.2), but is still useful in understanding general MooTools development; while the second is a new series that was created for the MooTools 1.3 release.
MooTools Essentials by Aaron Newton (2008, Apress), and MooTools 1.2 Beginner's Guide by Jacob Gube and Garrick Cheung (2009, Packt). Like the previous resources, these two are targeted towards MooTools beginners, but in book form. Both are really good introductions to the framework, but they are a little outdated since they still discuss MooTools 1.2.