A bibliography containing cites of all known reference books and how-to
manuals and also cites of selected technical articles on X and X programming
is regularly posted to comp.windows.x; it is ftp-able as
	ftp.x.org:/contrib/docs/Xbibliography.ps 
	ftp://gatekeeper.dec.com/pub/X11/R5-contrib/. 
	ftp://landru.unx.com/pub/X11/
The current maintainer is Steve Mikes, smikes%topgun@uunet.uu.net 
(smikes@unx.com).

Here is an unordered set of the reference books and tutorials most useful for
beginners; most appear on that list [comments are gathered from a variety of 
places and are unattributable]:

Asente, Paul J., and Swick, Ralph R., "X Window System Toolkit, The Complete
Programmer's Guide and Specification", Digital Press, 1990.  The bible on Xt.
A treasury of information, excellent and invaluable.  Distributed by Digital
Press, ISBN 1-55558-051-3, order number EY-E757E-DP; and by Prentice-Hall,
ISBN 0-13-972191-6. Also available through DEC Direct at 1-800-DIGITAL.  [The
examples are on ftp.x.org in R5contrib/ as asente-swick.examples.tar.Z.  They
were also posted to comp.sources.x as xt-examples/part0[1-5].]

Jones, Oliver, Introduction to the X Window System, Prentice-Hall, 1988,
1989.  ISBN 0-13-499997-5. An excellent introduction to programming with
Xlib.  Written with the programmer in mind, this book includes many practical
tips that are not found anywhere else. This book is not as broad as the
O'Reilly Xlib tutorial, but Jones is an experienced X programmer and this
shows in the quality and depth of the material in the book.

Young, Doug. "The X Window System: Applications and Programming with Xt
(Motif Version)," Prentice Hall, 1989 (ISBN 0-13-497074-8). The excellent
tutorial "X Window System Programming and Applications with Xt," (ISBN
0-13-972167-3) updated for Motif. Sources are on ftp.x.org in
R5contrib/young.tar.Z.  A Motif 1.2 version of this book is also out; see
ftp.x.org in contrib/book_examples/young.motif2.tar.Z.

Young, Doug and John Pew, "The X Window System: Programming and Applications
with Xt, OPEN LOOK Edition" (ISBN 0-13-982992-X). The tutorial rewritten for
OLIT, with new examples and drag/drop information. [Examples are in your
OpenWindows 3 distribution in $OPENWINHOME/share/src/olit/olitbook.]

Heller, Dan and Paula Ferguson. "Motif Programmers Manual".  The 6th volume
in the O'Reilly series covers application programming with Motif 1.2 and
earlier, including UIL; it's full of good examples (ISBN 1-56592-016-3).
Volume 6B is a reference book on Motif and UIL (ISBN ISBN 1-56592-038-4).
[The examples are available on uunet in the nutshell archives.]

Scheifler, Robert, and James Gettys, with Jim Flowers and David Rosenthal, "X
Window System: The Complete Reference to Xlib, X Protocol, ICCCM, XLFD, X
Version 11, Release 5, Third Edition," Digital Press, 1992. "The Bible" in
its latest revision, an enhanced version of X documentation by the authors of
the Xlib documentation. This is the most complete published description of
the X programming interface and X protocol. It is the primary reference work
and is not introductory tutorial documentation; additional tutorial works
will usually be needed by most new X programmers.  Digital Press order
EY-J802E-DP, ISBN 0-13-971201-1.

Nye, Adrian, "Xlib Programming Manual, Volume 1" and "Xlib Reference Manual,
Volume 2," O'Reilly and Associates.  The first volume is a tutorial with
broad coverage of Xlib, and the second contains reference pages for Xlib
functions and many useful reference appendices.  Both cover X11R5 (and R4).
ISBN 0-937175-26-9 (volume 1) and ISBN 0-937175-27-7 (volume 2).

Nye, Adrian, and Tim O'Reilly, "X Toolkit Programming Manual, Volume 4,"
O'Reilly and Associates, 1989, 1992. The folks at O'Reilly give their
comprehensive treatment to programming with the Xt Intrinsics, using the
Athena widgets in the examples; R5 versions are now available, as is a Motif
1.2 version (Volume 4M).

O'Reilly, Tim, ed.,  "X Toolkit Reference Manual, Volume 5," O'Reilly and
Associates. A professional reference manual for the X11R5 and X11R4 Xt.

Mansfield, Niall. "The X Window System: A User's Guide," Addison-Wesley,
1989.  A tutorial introduction to using X, now upgraded for R4. ISBN
0-201-51341-2.

Quercia, Valerie and Tim O'Reilly. "X Window System User's Guide," O'Reilly
and Associates. A tutorial introduction to using X. ISBN 0-937175-36-6.  
Covers R5; available in Athena and Motif editions.

Mui, Linda and Eric Pearce. "X Window System Administrator's Guide for X11 R4
and R5" [ORA Volume 8]. Help for X users and administrators.  ISBN
0-937175-83-8.

Drafts of John Ousterhout's book on TCL/TK are on sprite.berkeley.edu
(128.32.150.27) in /tcl. The final book was published by Addison-Wesley, ISBN
#0-201-63337-X.

(Prentice-Hall ordering is 201-767-5937. O'Reilly ordering is 800-998-9938
or 707-829-0515; ORA may also be contacted via email at order@ora.com or by
logging into gopher.ora.com as gopher.)

In addition, check the X11R4 and X11R5 core distribution in doc/tutorials for
some useful papers and tutorials, particularly the file answers.txt.  "Late
Night's Top Ten X11 Questions" by Dave Lemke (lemke@ncd.com) and Stuart Marks
(smarks@sun.com) answers other common questions and some of these here in
more detail.

A single volume, "Programmer's Supplement for R5" by David Flanagan, provides
an overview of new R5 features; it includes man pages for Xlib, Xt, and Xmu.
As of 10/93, its contents have been merged into other O'Reilly volumes, and
it is out of print.  [ISBN 0-937175-86-2]