- fiction
    - Abbott, Edwin A_
        - Flatland
            - Flatland - Edwin A. Abbott.epub
            - Flatland - Edwin A. Abbott.jpg
            - Flatland - Edwin A. Abbott.opf
    - Achebe, Chinua
        - Things Fall Apart
            - Things Fall Apart - Chinua Achebe.epub
            - Things Fall Apart - Chinua Achebe.jpg
            - Things Fall Apart - Chinua Achebe.opf

So in each directory that I use to delineate a library, I have a subdirectory for each author (in sort order form). Within each author subdirectory is a subdirectory for each book, with just the title, then the book with " - ."

I didn’t invent this, it’s just what Calibre spits out. When I buy a new book, I ingest it into Calibre, fix any metadata and export it to the NAS. Then I delete the Calibre library - I’m just using it to do the neatening up work.