XML
The W3C provides the official specification for the Extensible Markup Language (XML). They also have a succinct description.
The W3C also provides the official specifications for
Books about the XML language:
- Learning XML, 2nd Edition by Erik T. Ray (O'Reilly, 2003)
- Learning XSLT by Michael Fitzgerald (O'Reilly, 2003)
- XML Hacks by Michael Fitzgerald (O'Reilly, 2004)
- XML in a Nutshell, 3rd Edition by Elliotte Rusty Harold and W. Scott Means (O'Reilly, 2004)
- XPath and XPointer by John E. Simpson (O'Reilly, 2002)
- XQuery by Priscilla Walmsley (O'Reilly, 2007)
- XSLT, 2nd Edition by Doug Tidwell (O'Reilly, 2008)
- XSLT Cookbook, 2nd Edition by Sal Mangano (O'Reilly, 2005)
- XML and Web Technologies for Data Sciences with R by Deborah Nolan and Duncan Temple Lang (Springer, 2014)