XPath Reference
W3C XML Path Language Version 1.0 Specification
Although jQuery’s XPath support is limited, the W3C’s XPath Specification may still be useful for those wanting to learn more about the variety of possible XPath selectors:
TopXML XPath Reference
The TopXML site provides helpful charts of axes, node tests, and functions for those wanting to learn more about XPath:
MSDN XPath Reference
The Microsoft Developer Network website has information on XPath syntax and functions: