The book aims primarily to beginner MooTools developers, but may also be a good reference for users of previous versions willing to update their scripts.
That said, the book doesn't cover all of the methods offered by the library, but is intended by the authors as a step by step introduction to MooTools development and gives a solid knowledge of its undergoing philosophy.
The approach is strongly examples-oriented to better and clearly show how the library works and to give some starting point for developers willing to practice hands-on.
The Good Parts
The guide's table of contents spans from the core $ function to the plugin's usage and factoring, introducing the most relevant and commonly used part of the library. Writing is fluent and familiar keeping it easy while accurate.
I really appreciated the well done part about utility functions which are, in my opinion, one of the library's key features since they make possible to save lot of repetitive coding: functions like $type, $each or the Browser object grant a cross-browser and risk free development with little effort.
The other aspect which is deeply covered is the Class construction pattern.
By using this pattern every novice developer is able to code JavaScript classes with syntax similar to other programming languages like PHP, thus filling the lack of real classes in JavaScript and avoiding global object pollution.
A full chapter at the end of the book is dedicated to plug-in development.
This part has been an unexpected surprise, since few guides intended for technical audience teach code's design process with design sheets and documentation guidelines.
Beyond Examples
Talking about library presentation, it'd be a good idea to explain more in details some DOM manipulation methods, since many developers are used to circumvent (often too roughly) document and CSS limits by cutting and remixing elements with JavaScript.
MooTools has a big list of such methods which often are a better choice than native JS both for performance and browsers' support.
Conclusions
As a whole, MooTools 1.2 Beginners Guide gives a clear and solid basic knowledge of how the library works, making it possible for a novice developer to dive into the official documentation without feeling lost.
Even if some background thoughts about its development patterns would be appreciated, I'd definitely suggest the book to MooTools beginners (but not to JavaScript beginners for the mentioned reason).
Moreover it's a quick reference for commonly used methods and classes, therefore a book to keep on your desk.