The spec: http://www.w3.org/TR/xpath
http://xpath.alephzarro.com/content/cheatsheet.html#chXPath
DeepX ref at http://refcards.com/docs/deepx/xpath/XPath-1.0.pdf or http://www.digilife.be/quickreferences/QRC/XML%20Path%20Language%201.0.pdf
http://www.topxml.com/xsl/XPathRef.asp
CategoryCheatSheet
