TheServerSide.NET and Developmentor offer free download of the "Essential XML Quick Reference: A Programmer's Reference to XML, XPath, XSLT, XML Schema, SOAP, and More" book by Aaron Skonnard and Martin Gudgin (PDF version). 432 page book covers XML 1.0 and Namespaces, DTD, XPath 1.0, XPointer, XInclude, XML Base, XSLT 1.0
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