XML building blocks

What are some good XML specifications to get acquainted with? Here are the ones I find myself referring people to the most.

• Extensible Markup Language (XML) 1.0 (Fourth Edition). Close to a must-read, although XML basics are simple enough that it's often not necessary. Great to refer back to refresh whitespace and newline handling and which characters are allowed where.
• Namespaces in XML 1.0 (Second Edition). A must-read these days, as namespaces proved wildly popular and show up everywhere.
• XML Base. Very useful, especially with documents served over the Internet.
• XML Schema Part 0: Primer. Grok XSD.
• XML Schema Part 1: Structures. How to put schemas together.
• XML Schema Part 2: Datatypes. Again, this one proved popular beyond schema definitions, and is used to annotate typed XML in a bunch of other technologies.
• XSL Transformations (XSLT). Useful to go over once to play with a processor for a while, but the stuff I usually look up to answer questions is more often the next.
• XML Path Language (XPath). Again, another wildly popular building block, used to select, shred and transform documents the world over.
• XQuery 1.0: An XML Query Language. To be honest, I typically need this in the context of SQL Server, so I go to the highly awesome Books Online more often than not.

And there you go. File them away in a little "XML" folder in your favorites, and see how often to go back to them when you're working with XML-centric projects.

Enjoy!