There is a large amount of detailed information available about the XFree86 server, the clients that connect to it, and the assorted desktop environments and window managers.
/usr/X11R6/lib/X11/doc/README — Briefly describes the XFree86 architecture and how to get additional information about the XFree86 project as a new user.
/usr/X11R6/lib/X11/doc/RELNOTES — For advanced users that want to read about the latest features available in XFree86.
man XF86Config — Contains information about the XFree86 configuration files, including the meaning and syntax for the different sections within the files.
man XFree86 — The primary man page for all XFree86 information, details the difference between local and network X server connections, explores common environmental variables, lists command line options, and provides helpful administrative key combinations.
man Xserver — Describes the X display server.
http://www.xfree86.org — Home page of the XFree86 project, which produces the XFree86 open source version of the X Window System. XFree86 is bundled with Red Hat Linux to control the necessary hardware and provide a GUI environment.
http://dri.sourceforge.net — Home page of the DRI (Direct Rendering Infrastructure) project. The DRI is the core hardware 3D acceleration component of XFree86.
http://www.redhat.com/mirrors/LDP/HOWTO/XFree86-HOWTO — A HOWTO document detailing the manual installation and custom configuration of XFree86.
http://www.gnome.org/ — Home of the GNOME project.
http://www.kde.org/ — Home of the KDE desktop environment.
http://nexp.cs.pdx.edu/fontconfig/ — Home of the Fontconfig font subsystem for XFree86.
The Concise Guide to XFree86 for Linux by Aron Hsiao; Que — Provides an expert's view of the operation of XFree86 on Linux systems.
The New XFree86 by Bill Ball; Prima Publishing — Discuses XFree86 and its relationship with the popular desktop environments, such as GNOME and KDE.
Beginning GTK+ and GNOME by Peter Wright; Wrox Press, Inc. — Introduces programmers to the GNOME architecture, showing them how to get started with GTK+.
GTK+/GNOME Application Development by Havoc Pennington; New Riders Publishing — An advanced look into the heart of GTK+ programming, focusing on sample code and a thorough look at the available APIs.
KDE 2.0 Development by David Sweet and Matthias Ettrich; Sams Publishing — Instructs beginning and advanced developers in how to take advantage of the many environment guidelines required to built QT applications for KDE.