Docker (Recommended method):
Docker Image by linuxserver.io, thanks guys!
Text Guide | Video Guide
Download war file
Installation script / Control Panel & Installer (BETA)
Instructions for Linux coming soon but if you are running Linux I am guessing all you need to know is "java -jar booksonic.war -Dserver.port=4040", yup, it's as simple as that now.
Want to install Booksonic in the cloud instead of your own hardware?
I recommend renting a Contabo VPS and installing booksonic there. Currently the first month is free for all plans and then it starts as low as 3.99 euro for 300GB storage and 4GB ram. If you got a huge library you can get as much as 1.4TB storage for 12.99 euro
Obs, if you click the contabo links and rent a server I will receive a small commision at no extra cost to you, this has not influenced my decision to recommend them in any way, they are what I use personally and what the Booksonic demo is hosted on.
Build from source (Free): Link
This option is totally free but you will need to re-build it if you want to update it. This one is recommended if you want to make changes to the app or don't have the play store on your phone.
Google Play ($3): Link
This one is recommended if you just want to use the app and never have to worry about updating it since it comes with automatic updates