Though this author has a nice conversational writing style (rare for technical authors), this book is little more than a run-through of all the menu commands and fields in the ModX Manager UI (in the order they appear--not necessarily in the order you'd use them), which is the absolute worst way to teach complex software. There are brief explanations of the various components to the CMS, but without the context a start-to-finish simple example site would have provided, they are overwhelming.
If you're looking for a reference book for what a specific menu command does, or what a specific field is for, this book is for you.
If, on the other hand, you have a site to build (with fully designed HTML and CSS ready to go), and simply need to know how to put it into ModX, look elsewhere.
This book sorely needs an early chapter with a simple tutorial on getting a basic site set up. Only after that would the minutia of the Manager UI be helpful, meaningful or interesting.