Also by "reader apps" I mean "reader apps with their walled gardens", like Kindle, as opposed to readers for local files. Boox's builtin reader is OK, but I've used other e-readers whose builtin reader was worse and it'd be useful to have a 3rd party one for your local files. Ironically those e-readers are not Android-based.

I know you can de-DRM Kindle books, but there are other walled gardens that don't allow you to do that easily (for me it's some Chinese e-book apps).