Resources for learning about file system implementations?

For starters? Him because he is 'the father' of the *BSD filesystems:

https://en.wikipedia.org/wiki/Marshall_Kirk_McKusick

That's all online via the web, as PDF, annotated and whatnot.

For 'something else': https://en.wikipedia.org/wiki/Be_File_System

As living on via Haiku, and documented there under 'Other Resources':

https://www.haiku-os.org/documents/