...I ran into Dave Pawson's XSL FAQ, which gave me the ideas I needed to finally crack it (it turns out I was fairly close...). I strongly recommend his site for anyone serious about XSLT and XPath; he's got some really cool examples on it.  [Commonality]

Jeni Tennison's site is also very good, it's mainly an index of her postings to the XSL-List.  Any time I have to do a grouping problem, I run to her site.  She's got the best explanation of grouping I've ever found.  Grouping seems like an easy thing until you try it, but it can be a mind bender.