An online version of this presentation. A complete tutorial (http://nwalsh.com/docs/tutorials/xml99/) is also available.
Ken Holman offers a number of commercial tutorials
XML Path Language 1.0, W3C Recommendation
XSL Transformations 1.0, W3C Recommendation
Extensible Style Language, W3C Working Draft
Extensible Markup Language 1.0, W3C Recommendation
Namespaces in XML 1.0, W3C Recommendation
James Clark's XSLT processor
Scott Boag's XSLT processor
James Tauber's open source XSL formatter and renderer; see also http://xml.apache.org/ (http://xml.apache.org/)