This presentation: http://www.ibiblio.org/xml/slides/xmlonesanjose2001/xlinks/
XLink Specification: http://www.w3.org/TR/xlink/
Chapter 19 of the XML Bible, 2nd Edition: http://www.ibiblio.org/xml/books/bible2/chapters/ch19.html
Chapter 10 of XML in a Nutshell