KOReader is a cross-platform e-book reader.
It has a sync server available to sync book progress between devices, but this has not been updated for 5 years (plus who writes Lua these days anyways?). There is another implementation by myelsukov, but it is very minimal and, according to its author, "quick and dirty."
I hope to build an alternative that uses a proper database (SQLite/MariaDB) and is scalable. I also wanted something pre-configured to run in Docker.
The core API of this project is loosely based on the interface in myelsukov's application. It is still heavily work-in-progress and not ready for use yet.