Arguably

https://play.google.com/store/apps/details?id=biz.bookdesign...

should ~~be~~ EDIT have been ENDEDIT opensource --- it does at least work to support Project Librivox (or at least that's my understanding)

Seems to no longer be available (see below)

They do exist, since a pretty long time. I bought a pocketbook beginning 2010s. https://pocketbook.ch.

If you mean epub reader software Calibre and a bunch of others exist since pretty much the beginning of epub