If you are interested in learning more about XML, and if you are a fan of O'Reilly books (as I am), you will want to check out Learning XML by Erik T. Ray and XMLinaNutshell by Elliotte Rusty Harold and W. Scott Means (both published by O'Reilly in 2001).
The growth and development of XML is well documented online in resources such as the following:
The World Wide Web Consortium's official web site is the best place to go for the latest news on new XML standards and proposals.
XML.com, part of the O'Reilly network, is a clearinghouse of great articles and information on XML.
The following sites have comprehensive information regarding XML:
The XML FAQ: http://www.ucc.ie/xml/
The XML Cover Pages: http://www.oasis-open.org/cover/xml.html
XML INFO: http://xmlinfo.com
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