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I have a favorite that is the definitive reference.  To my perspective
it offers a reasonable introduction but also goes into the full
details of the Intrinsics.  When I started using it I was already
familiar with Xt and the concepts behind it, so newcomers may or may
not find it useful.  I've always found it accurate and complete, which
means it's 1000 pages.

Asente, Paul J., and Swick, Ralph R., "X Window System Toolkit, The
	Complete Programmer's Guide and Specification", Digital Press,
	1990, 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 other book I commonly recomend to novices is:

Young, Doug. "The X Window System: Applications and Programming with
	Xt (Motif Version)," Prentice Hall, 1989 (ISBN 0-13-497074-8).
	(ISBN 0-13-972167-3)

And of course O'Reilly has an entire series of manuals on X and Xt.
O'Reilly ordering is 800-998-9938.  In particular, Volume 5 is an Xt
reference done in manual page style.  The 3rd edition is extensively
overhauled and goes far beyond the MIT manual pages.  I'm finding it
very useful.  In particular, the permutted index and references to
other manual pages help a great deal in chasing down related
information.

I read two periodicals, "The X Resource" and the "The X Journal".
These are the only two dealing specifically with X.  "The X Resource"
is published quarterly, by O'Reilly, with one of the issues being the
MIT X Consortium Technical Conference Proceedings.  There is no
advertising.  I've found it informative with pretty good depth.  For
orders, call 1-800-998-9938, or email cathyr@ora.com.  For editorial
matters, email adrian@ora.com.  Table of contents are posted at
math.utah.edu in ~ftp/pub/tex/bib in TeX form and on ftp.uu.net in
~ftp/published/oreilly/xresource in ASCII form.


"The X Journal" is a bimonthly trade rag with lots of advertising.
The articles are informative and oriented toward a less technical
audience.  I read it more to see what's going on then with an
expectation of learning a great deal (but remember, I represent a
fairly small percentage of people).  Also, they have a pretty good
collection of people on the advisory board and as columnists.  Call
(908) 563-9033.

For German readers try:

	Brede et.al. "Programmieren mit OSF/Motif"
	Addison-Wesley, Germany (ISBN 3-89319-211-5)

It is a book which introduces Xt technics with motif widgets from an
applications programmers point of view (no widget design stuff).