XML

• A Technical Introduction to XML
  A good informal description of the XML specification - covers tags, attributes, nesting, and DTDs.
• XML Specification (W3C Recommendation)
  The most authoritative (albeit more difficult to read) source containing latest updates to the standard.
• The XML Cover Pages
  Everything you wanted to find out about XML and related topics
• WWW Consortium Technical Reports and Specifications
  The official place to go for latest of all XML-related standards
• XML gets down to business
  A high-level overview of XML and how it encodes semantics (as compared to HTML). Describes DTDs and how they constrain and define the tags and attributes usable within an XML document. Briefly covers the debate over XSL (as it competes with CSS and DOM.) Mentions a few commerce-related applications of XML, such as micropayments and ads.
  In "NetWorker: The Craft of Network Computing", Sept 1999.
  
• "Data on the Web: From Relations to Semistructured Data and XML",
  by Serge Abiteboul, Peter Buneman, Dan Suciu, and Jim Gray.
• Practical XML with Linux
  A well-written CNN article
• A very brief overview of XML
  Contains pointers to related articles
• The XML Bible (Updated chapters)
• XPointer Specification (W3C Working Draft)
  Explains how to link between and into XML documents

XML with XSL

• XSL Specification (W3C Working Draft)
  Explains how to use extensible stylesheets to specify:
  1) transformations between documents;
  2) an XML vocabulary for specifying the formatting of XML documents.
• Introduction to XML/XSL
  Contains many XML/XSL examples walking through the features of stylesheet transformations. Cover a good range of topics, while being more accessible than the XSL specification.