Learning XML requires a basic understanding of WWW, HTML, and Web scripting languages such as JavaScript or VBScript. Many of the following articles were written around the time that XML first started to make a splash in the development world. But the information is still valid. Start with the fundamentals and move down the list to more sophisticated concepts.

Beginner:
Project Cool’s XML QuickStart
An Introduction to XML for Java Programmers
XML: Code Format of the Future?

Intermediate:
Mozilla’s Potpourri of Rendered XML
Multilingual Web Pages with ASP and XML
Take Advantage of XML Using VB and ASP

Advanced:
Compressing XMLPart I, Writing WBXML
Compressing XMLPart II, Server-Side WBXML
Serve Business Graphics from Any XML Source
Implement a One-Stop Authentication Web Service with .NET
Build an XML Based Scheduling Utility

XHTML:
Convert XML Documents into Different Formats with the XSL Template Language
Convert a Text File to XML
Take a Lesson from a Class Act
Transform Your Data With XSL
Generate PDF Files Dynamically Using XSL-FO
Shrinking Code With the XML Parser

DTDs and XML Schema:
Generating XML From ADO Recordsets
Applying XML Schema to XML Documents
The Tao of Recursion: Named Templates in XSLT
Converting Fixed-Width Text Records to XML
Convert Schemas to Documents

XPath & XQuery:
Introduction to XQuery (A Four Part Series)
Integrating XML into ASP.NET Web Sites

Classes:
DevX eLearning: Introduction to XML 1.0, Second Edition
DevX eLearningL: XML 1.0 Series, Second Edition
DevX eLearning:Technical Web Development Package
Introduction to XML

Good Links:
W3C.org
O’Reilly XML.com
XML.org
The Cover Pages
W3Schools: XML Tutorial

Discussion Groups:
DevX xml.general Discussion Group
MSDN: XML Core
The Cover Pages: SGML/XML Discussion Groups and Mailing Lists
TopXML Online Discussions

Books:
Learning XML, by Erik T. Ray.
Get the Basics on “XHTML”, by Chuck Musciano and Bill Kennedy.