The LearnList

www.w3schools.com/xml/default.asp
www.topxml.com/xml/LearnXML.asp
www.tutorialized.com/tutorials/XML/1
www.hotscripts.com/XML/Tips_and_Tutorials/index.html
www.alistapart.com/topics/code/xml/
www.xml.com/pub/a/98/10/guide0.html
www.xmlfiles.com/xml/
www.tizag.com/xmlTutorial/index.php

Sams Teach Yourself XML in 24 Hours: Complete Starter Kit
XML in a Nutshell
XML in Easy Steps (In Easy Steps)
XML for the World Wide Web (Visual QuickStart Guides)