Nobody has any reason to poke around in X at this level (at least not with any sanity).

If you do, you'll soon understand the horrors of X11 (I assume that you already understand the good stuff if you even contemplate this book).

Granted, if you really, really want to understand X at a very, very low level (to implement something new and hopefully better), this book can be of use. Otherwise, it is a technical manual that any application programmer should steer clear of (or any aspiring UNIX/LINUX programmer/user/administrator), so as not to be scared away. There exists toolkits and other stuff that makes programming under X11 a breeze, but this will with 99% probability not help you!