All of the W3C recommendations are listed at
http://www.w3.org/TR/
The XML.org site is also pretty comprehensive:
http:www.xml.org
Zvon has a full range of XML tutorials at:
http://www.zvon.org/
The chapters on XSLT and XSL-FO from The XML Bible are on line at:
http://www.ibiblio.org/xml/books/bible2/chapters/
The most comprehensive reference book is Michael Kay's XSLT Programmer's Reference (second edition), which is published by Wrox. What it lacks in pedagogy, it makes up for in its encyclopedic approach.
The W3C XSLT specification is online at:
http://www.w3.org/TR/xslt