Documentation I Have Known And Loved
Modern Texts
More-or-less accurate descriptions of the way X works now.
The Cygwin/X developer site has many useful bits.
The DRI documentation. Mostly still applicable!
- XFree86 DDX documentation:
DESIGN doc.
Additional XFree86 DDX developer documents
The protocol & API specifications for the latest X11 katamari release, covering pretty much everything.
Ancient Scrolls
A miscellaneous collection of documentation surrounding the technical side of X, in particular what it gets wrong and the way it's been implemented in the past. Those who do not learn from history are doomed to reinvent it.
An LBX Postmortem. Packard, 2001.
Component Design Specification for the MIT Multi-Threaded X Window Sample Server. Haynes, et al., 1993. Available in three parts from the R6.0 source, mirrored here. Consider this a warning about how not to multithread an X server.
D11: a high-performance, protocol-optional, transport-optional window system with X11 compatibility and semantics. Kilgard, 1995.
The Evolution of the X Server Architecture. Packard, 1999.
Why X Is Not Our Ideal Window System. Gajewska, et al., 1990.
X Server Multi-rendering for OpenGL and PEX. Kilgard, et. al, 1994. Be glad you don't have to care about PEX anymore.