In order to take full advantage of MooTools, you should check out these excellent resources:
MooTools Docs (http://mootools.net/docs/) contains the official MooTools documentation. Created by the MooTools team, it's the most comprehensive online resource for MooTools syntax and usage. As you write in MooTools, you'll find that this is a handy resource to have open for quickly finding references to particular MooTools code.
The MooTools Users Google groups (http://groups.google.com/group/mootools-users) is a discussion group on the topic of MooTools. You'll find a variety of topics here and if you run into specific MooTools problems, it's a great place to seek help.
mooforum (http://www.mooforum.net/) is the unofficial forum for MooTools. If you're looking for help on specific things or simply want to interact with other MooTools developers, you should sign up for a free user account.
The MooTorial (http://mootorial.com/wiki/) is an online wiki-style tutorial on the MooTools framework written by Aaron Newton. It's a good starting point for learning the basics of the MooTools framework and is a great online supplement to the things you learn throughout this book.