The Xlib Programming Manual and Reference Manual would probably be of interest, they are getting a bit old now but xlib has not changed much and they are still useful.
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You can access those manuals here
https://www.x.org/wiki/ProgrammingDocumentation/
They are the O'Reilly Ones
Shame that Volume 1 is missing, it is the most important one and the only one that is really still relevant. Luckily these are not hard to get, I got Volume 1 & 2 for $6 used not too long ago and there were plenty more copies, oddly package deals of both Xlib volumes were cheaper than than just Volume 1 on its own. Have learned a good deal from it.