The definitive and complete resource is Michael Kay’s XSLT book (originally Wrox Press, but now part of O’Reilly I think?). Granted, a paper book as a resource isn’t great, but it’s some book!

Kay went to Cambridge (Trinity) and did his PhD under Sir Maurice Wilkes. He’s no joke: one of the examples from the book is implementing the Knight’s Tour. Although this is only the chapter pre-amble, the commentary is indicative of what kind of book this is:

https://www.oreilly.com/library/view/xslt-20-and/97804701927...

It’s like Dijkstra writing the definitive reference manual for CSS and one of the formatting examples being an engine for solving Towers of Hanoi.

While I have qualms about the utility of XSLT as a general purpose language — it’s fine if you embed it in an imperative language a la SQL in Python, and awful if you try and use it standalone — it also gave us XPath which is by far the best way of querying markup that I’ve ever used. (Well, until CSS got attribute selectors, so I use it less and less these days I suppose.)

(You probably know all this, but it was interesting to add to the thread.)