Custom album.nfo is not working

[TheDreadPirate]

Any reason you are placing audio books in a music library and not a book library? A book library does accept audio files and will track progress while a music library will not.

Regarding your question about NFOs. I've never used them with audio so I am not sure how a books library, or a music library, will interpret NFOs.

What you can experiment with is removing the NFOs (back them up) and asking Jellyfin to generate NFOs so you can compare how Jellyfin uses NFOs in this context vs what you've created.