Xlib is the C interface to the X window system. It is the basic
substrate upon which other interfaces to X (such as Tcl/Tk) are based. The two-volume set of manuals
by Nye provide a comprehensive reference.
Adrian Nye, ed., Xlib Reference Manual (3rd edition),
O'Reilly & Associates, Sebastopol, CA 1992.
Adrian Nye, Xlib Programming Manual (3rd edition),
O'Reilly & Associates, Sebastopol, CA 1992.