Although a bit superficial technically speaking X Power Tools pieces together a survey of generally useful X11 knowledge in a quick and easy read. Think of this book as a good place to begin an exploration of X. For those who know next to nothing X11 or windowing systems, and have little experience configuring or running Linux the book is a perfect introduction (5 stars). For those who have a lot of experience configuring or running Linux, who certainly have had to interact with X before, much of this material will already be familiar (3 stars). I fall into the latter category and still found the book useful.
Here is a list of the "x" commands on my linux box cross referenced with the book's index. Note this isn't necessarily a complete list and the book is more than just a command reference, as it goes into detail explaining configuration file formats and options. Still this list gives an idea of the scope of the book.
x11perf NO
x11perfcomp NO
xargs NO
xauth YES
xbiff NO
xbmtopbm NO
xbsh NO
xcalc YES
xcam NO
xclipboard YES
xclock YES
xcmsdb YES
xconsole NO
xcursorgen NO
xcutsel NO
xdfcopy NO
xdfformat NO
xdg-desktop-icon NO
xdg-desktop-menu NO
xdg-email NO
xdg-icon-resource NO
xdg-mime NO
xdg-open NO
xdg-screensaver NO
xdg-user-dir NO
xdg-user-dirs-update NO
xditview NO
xdpyinfo YES
xdriinfo NO
xdvi NO
xdvi.bin NO
xdvi-xaw.bin NO
xdvizilla NO
xedit NO
xev NO
xeyes NO
xfd YES
xfontsel YES
xfsinfo NO
xft-config NO
xgamma YES
xgettext NO
xhost YES
ximtoppm NO
xinit YES
xinput NO
xkbbell NO
xkbcomp YES
xkbevd NO
xkbprint YES
xkbvleds NO
xkbwatch NO
xkill YES
xload NO
xlogo NO
xlsatoms NO
xlsclients NO
xlsfonts NO
xmag YES
xman NO
xmessage NO
xmkmf NO
xmodmap YES
xmore NO
xpcshell-1.9 NO
xpmtoppm NO
xprop NO
xqxdecode NO
xrandr YES
xrdb NO
xrefresh YES
xscanimage NO
x-session-manager NO
xset YES
xsetmode NO
xsetpointer NO
xsetroot YES
xsm YES
xstdcmap NO
xsubpp NO
xterm YES
x-terminal-emulator NO
xtrapchar NO
xtrapin NO
xtrapinfo NO
xtrapout NO
xtrapproto NO
xtrapreset NO
xtrapstats NO
xulrunner NO
xulrunner-1.9 NO
xvidtune NO
xvinfo NO
xvminitoppm NO
xwd YES
xwdtopnm NO
x-window-manager NO
xwininfo YES
xwud YES
x-www-browser NO
xxd NO