Also, found this tidbit while researching and remembered you said you used Ubuntu
Enable forwarding with UFW
nano /etc/default/ufw
Replace:
DEFAULT_FORWARD_POLICY="DROP"
With:
DEFAULT_FORWARD_POLICY="ACCEPT"
Then
ufw reload
Simply head on over to YouTube and search for set up emby server. You can also go to https://emby.media/community/index.php which is the Emby forum and ask questions. Great support. Anyway during the setup of a new library you can choose what sort of media it is..... Movies, music, audiobooks, etc.
As far as I know it does not, sadly. You can look for a airsonic compatible web app player (https://airsonic.github.io/docs/apps/. If it supports bookmarks, I believe it should work with booksonic progress sync