That book is extremely useful and relevant. The author also has a great website which is a treasure trove of information
https://www.amazon.com/s/ref=assoc_res_sw_top_seller?search-alias=aps&tag=bobsgui-20&linkCode=w13&linkID=UGZVC3A5T2IIHMI4
. Completely recommend buying that book.
We have also used MODX: The Official Guide by Bob Ray when starting out with Modx. Recommended. It was very useful and nicer to have in book form for reference than trawling around the web. I'm not sure if there is / will be a revised edition as our book covers Modx from a few versions ago of Revo. A lot is still the same, however.
There is the additional advantage that Bob prowls these forums with many positive contributions and solutions.
The
Digital Edition of the book is a little more current. For both
versions, the chapter on the MODX Manager is out of date since the Manager has been reorganized somewhat, though it's usually not too difficult to find things. The other chapters are generally still accurate.