Favourite Book Reader now works on iOS.
Major e-book formats are ePub (including main features of ePub3), unencrypted Kindle azw3 (a.k.a. mobipocket) and fb2(.zip).
Other supported formats: RTF, doc (MS Word), HTML, plain text.
FBReader can synchronise your library, reading positions, bookmarks, etc., with the FBReader book network (https://books.fbreader.org/), a Google Drive™ based cloud service.
Alternatively, you can download books in the browser and open them using FBReader.
A rich set of options allows you to configure reading parameters and app behaviour for your preference.
The free version of FBReader has a few limitations. You cannot store more than ten books in the library. The built-in online translation feature is disabled. You can upgrade FBReader to the full version to unlock all the features.