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Plex works fine, and with clients like Prologue it's awesome:

Not an audiobook organizer, but I use EasyTAG to tag my audiobooks before importing them to Cozy: https://wiki.gnome.org/Apps/EasyTAG/

I use Jellyfin as an open source alternative to Plex. I personally don't have any audio books on my home server, but it has support for them.

Oh really? I use Jellyfin also, but I never knew it supported audiobooks!

I ended up using m4b-tools to organize and tags files in folders with Plex as front end and Prologue (iOS) as my preferred player.