For XML learning, the basic definition documents and tutorials available from the W3C are a good first step. You can browse the available documents at this Web site:
http://www.w3.org
The following book is an excellent reference to XML and related technologies:
Essential XML Quick Reference: A Programmer's Reference to XML, XPath, XSLT, XML Schema, SOAP, and More by Aaron Skonnard and Martin Gudgin, published by Addison-Wesley (ISBN: 0201740958)
Progress Software's Education Services also offers two in-depth classes that are available over the Internet:
XML Essentials
OpenEdge Development with XML
Information about these classes can be found at this address:
XML Essentials