Some of the most useful information on the X library can be
found in the include files Xlib.h and X.h. These files are usually
found in /usr/include/X11 or /usr/include/X11R5.
There are several example programs in this document. More comprehensive
examples are in /export/Examples/X on the CUED
teaching system and in the X source tree (/usr/src/X11R4
on the CUED teaching system). Even more examples are available via
ftp at
ftp.uu.net:/published/oreilly/xbook/xlib
Manpages for the routines are online.
The online help facility at CUED contains a file of answers to frequently asked questions and some discussion of advanced topics.
Relevant newsgroups are:- comp.windows.x, comp.sources.x