For just learning XML, I totally recommend the quickstart guide. I had an old edition, of about 180 pages, and loved it. Now I see it's really bulked up in size. I'm I'm doing work with XML, I always keep it handy. It's just a great book with lots of easy code to read and work with:
XML for the World Wide Web (Visual QuickStart Guide) by Elizabeth Castro As far as IBM goes, they recommend
XML in a Nutshell by Elliotte Rusty Harold, W. Scott Means and
The XSLT Cookbook, although, personally, for XSLT, I really like
Beginning XSLT 2.0: From Novice to Professional, although you've got to be careful of the version if you're interested in taking the exam.
-Cameron McKenzie
[ January 11, 2007: Message edited by: Bear Bibeault ]