This app was mentioned in 5 comments, with an average of 1.40 upvotes
Until the day this feature gets added properly you can try https://booksonic.org/. It's selfhosted audiobook software. It has a companion Android app as well.
Put booksonic and plex together behind Organizr and it could feel like one system.
how about this one
I'm using Booksonic to stream to my phone, but it's a lot to set up. Edit: https://play.google.com/store/apps/details?id=github.popeen.dsub
I would check out Booksonic not free and requries a little bit of tech know how but works https://play.google.com/store/apps/details?id=github.popeen.dsub