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.