Most ebook readers (with the exception of Amazon's own Kindle reader) can read virtually all of these, though some will require additional extensions. E.g., FB Reader <https://fbreader.org/>, PocketBook Reader <http://reader.pocketbook.digital/eng>, Onyx's Neoreader (BOOX) <https://onyxboox.com/> ... no indepedent download AFAIU, KOReader (Kobo)<https://www.kobo.com/us/en/p/apps>, Moon+ Reader, etc.
Many of these are designed for tablets or e-ink devices, though you'll also find desktop/laptop versions of some software (e.g., FBReader).
F-Droid ebook readers: <https://search.f-droid.org/?q=ebook+reader&lang=en>
