I've been using XSLT 2 for many years and I decided I need to get up
to speed on XSLT 3.0 for various reasons.

Surprise! No books on it that I have found. I've depended on the 4th
edition of Michael Kay's XSLT 2.0 etc for a long time, but he's
apparently not doing a new one. There is nothing on XSLT 3.0 on Safari
books (or whatever O'Reilly calls it now).

I hypothesize from this that there are very few people coming to XSLT
from the ground up these days, only old farts updating their skills.

Anyway, I can use google or something and look for information, but
I'd rather get info from this list. What are some good references on
XSLT 3.0?

There are the official specs on XSLT 3.0, XPath 3.1, XPath 3.1 functions
and operators, if you go schema-aware the XSD specs.

There are the Saxon documentations which contain a good part of
reference documentations of XSLT instructions, XPath functions and
certainly anything new in XSLT 3, like streaming, maps, arrays.

Some people I don't know have build what goes as an XSLT 3.1 reference
online https://xsltdev.com/.

As for tutorials and explanations, Dimitre has courses on Pluralsight,
Ken on some other site (Udacity?).

Then there are talks/papers by Norm
https://www.balisage.net/Proceedings/vol25/print/Walsh01/BalisageVol25-Walsh0
1.html
and Jirka https://www.kosek.cz/xml/2019xmlss/Kosek_XSLT4DailyCoding.pdf