XML in a Nutshell, second edition
Elliotte Rusty Harold and W. Scott Means
O'Reilly & Associates, 2002
ISBN 0-596-00292-0
XPath: http://www.oreilly.com/catalog/xmlnut/chapter/ch09.html
XML Bible, second edition
Elliotte Rusty Harold
Hungry Minds, 2001
ISBN 0-7645-4760-7
Chapter 17 XSL Transformations: http://www.cafeconleche.org/books/biblegold/chapters/ch17.html
This presentation: http://www.cafeconleche.org/slides/sd2003west/handsonxslt
XSL Transformations 1.0 Specification: http://www.w3.org/TR/xslt/
XPath Specification: http://www.w3.org/TR/xpath