A look at my personal Plex Audio Book library setup.
Audiobooks (Audible) metadata agent
https://github.com/macr0dev/Audiobooks.bundle
How To listen to Audiobooks With Plex
https://www.thestreamingadvisor.com/how-to-listen-to-audiobooks-with-plex/
Plex & Booksonic Audiobook Guide
https://github.com/seanap/Plex-Audiobook-Guide#tags-that-are-being-set
