X Protocol Reference Manual, Vol. 0: For the X Window System, Version 11 (The Definitive Guides to the X Window System) by Robert Scheifler

Volume 0, X Protocol Reference Manual provides a complete reference to the X Network Protocol, which underlies all software for version 11 of the X Windows System. The X Protocol is the language in which computers communicate information between the X server and X clients, whether the server and clients are operating on different systems in the network or on the same system.