There is a large amount of detailed information available about the X 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 xorg.conf — Contains information about the xorg.conf configuration files, including the meaning and syntax for the different sections within the files.
man X.Org — The primary man page for X.Org Foundation information.
man Xorg — Describes the X11R6.8 display server.
http://www.X.org/ — Home page of the X.Org Foundation, which produces the X11R6.8 release of the X Window System. The X11R6.8 release is bundled with Red Hat Enterprise Linux to control the necessary hardware and provide a GUI environment.
http://xorg.freedesktop.org/ — Home page of the XR116.8 release, which provides binaries and documention for the X Window System.
http://dri.sourceforge.net/ — Home page of the DRI (Direct Rendering Infrastructure) project. The DRI is the core hardware 3D acceleration component of X.
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 X.
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 on taking advantage of the many environment guidelines required to built QT applications for KDE.