A challenging book. The challenge is to find the differences with the book "The Joy of X: An Overview of the X Window System", by Niall Mansfield, published in 1992. These two books have the same number of pages, each page starts and finishes with the same words, the diagrams are the same, the words on each page are the same. They are the same book silently rebadged.
The text of the book finishes by saying "X is a dynamic and rapidly expanding area...". Things have changed in the eighteen years between these publications. For example, X is now managed through Xorg, not The MIT Consortium, but there is no mention of Xorg in the current book. The table of X releases on page 121 is badly out of date. A major user of X is Linux, but there is no direct mention of Linux. The Gtk toolkit is possibly the most commonly used X toolkit today, but it does not get mentioned. There are many other problems. Unfortunately, the poor example of representative Xlib programming (pp 134-5) remains unchanged. This current book needs major revisions to update its information. However, the basic information is well presented and well worth having for reference.
So, if you have a copy of the 1992 book, you will be disappointed with this one as it is no update on the original.