I've yet to see a book on X Windows that didn't look as if it were written at gunpoint to make a deadline and a dollar.
The O'Reilly series seems to be the accepted reference:
"The Definitive Guides to the X Window System", Volume One: Xlib Programming Manual for Version 11 by Adrian Nye. O'Reilly & Associates 1992, ISBN 1-56592-002-3
And so on for half a dozen volumes.
X Window System Programming, by Nabajyoti Barkakati (SAMS Macmillan 1991, ISBN 0-672-22750-9) is perhaps a bit better as a tutorial.
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