This presentation: http://www.cafeconleche.org/slides/sd2002east/xlinks/
XLink Specification: http://www.w3.org/TR/xlink/
Chapter 19 of the XML Bible, 2nd Edition: http://www.cafeconleche.org/books/bible2/chapters/ch19.html
Chapter 10 of XML in a Nutshell, 2nd edition