Booksonic is a Subsonic client with the main goal of being as good as possible at streaming audiobooks.
With it you can connect to your server and listen to your audiobooks from wherever you are, perfect for those boring bus rides.
Booksonic has a couple of features that is unique for it when compared to other Subsonic apps.
These are things like
Book descriptions, either you add your own to the server or the app will try and look it up online.
Author, narrator and book length are displayed on the book view.
Sleep timer, to reset the countdown just shake the phone.
Tracks are marked as heard so you know what you have listened to.
Import/Export server details and heard status
Support for almost all audio formats, including m4b files (as long as there is no DRM protection)
More will be added as I think of them.
Other features include:
Variable playback speed (Marshmallow+)
Multiple themes to choose from
Browse by tags or by folder structure
Wear Support (Lolipop+)
And a lot more.
These are part of the app Booksonic is based on, it's called DSub. If you are intending to use the app for music then you should go for DSub instead or use both because essential music features (like shuffle) has been removed from Booksonic.
Booksonic will work with any Subsonic server but to get the most out of it your server need to be a Booksonic Server. You can get the server for free at http://booksonic.org
Both the app and the server are released under GPLv3.