So here's the thing: the internet is full of tutorials about specific implementations, but what you really want is the core concepts.

You want to master (or at least understand at a basic level) the core concept so that you can then adopt any technologies.

In that regard, my best find was "LDAP for rocket scientists" -- https://www.zytrax.com/books/ldap/ .

It's long and boring, but immensely worth it. I didn't really understand the why of LDAP until I read that guide.