4.4. Additional Information on XPath

The primary source of information on XPath is the reference [6] but other web sites and books are also interesting

http://www.w3.org/Style/XSL/
is the best starting point to get information on XSL (including both XPath 1.0 and XPath 2.0) with links to tools and tutorials.
XSLT 2.0 and XPath 2.0 Programmer's reference (4e ed.) [27]
A complete and didactic description of all aspects of XPath and XSLT.