Ah yeah I forgot I was running the bleeding version, either way it will probably be stable soon ish

Findroid also recently added this

Not sure if it made it into this release or not, but transitioning support is in the bleeding version, so this won’t be the case for long

There is actually an open source eReader, openbook https://www.oddlyspecificobjects.com/projects/openbook/

Unfortunately, the current screen it uses is not being made anymore. People have stated that they have gotten other screens to work, but no official instructions on using them yet