I do a lot of work on building solutions around XSL and have a set of reference links that I live by so I thought I would pass these along. These reference sites are the ones that I always give to new developers or design partners starting out in XSL.
ZVON XSLT Reference
ZVON XSL:FO Reference
XSLT Questions and Answers
Standard XSLT extensions, useful until we all move to XSLT 2.0