This presentation: http://www.cafeconleche.org/slides/sd2002east/xlinks/
XPointer specifications:
XPointer Framework: http://www.w3.org/TR/xptr-framework/
XPointer xmlns() Scheme:http://www.w3.org/TR/xptr-xmlns/
XPointer element() Scheme: http://www.w3.org/TR/xptr-element/
Chapter 20 of the XML Bible, 2nd edition: http://www.cafeconleche.org/books/bible2/chapters/ch20.html
Chapter 10 of XML in a Nutshell