The X Window system

• The Xlib Programming Manual
• A short Xlib tutorial
• A C++ program to generate every possible X11 request
• The GWM [Generic Window Manager] manual

X Inter-Client Communication

• The ICCCM [Inter-Client Communication Conventions Manual]
• Andrew Taylor on X Window Manager Communication

More documentation and reference material

• A nicetutorial

The Motif Zone	Kenton Lee's	motifdeveloper.com

See also: Computer Graphics.