And, yes, I've tried the third-party app Book Reader but, sorry guys, it has no UI and really isn't ready for prime time.
I too would love to hear an explanation of how it has no UI and isn't ready for prime time.
Are you sure you have the right one? There is 'PlayEpub Book Reader' and 'Book Reader for EPUB and Kindle'. The latter is considered the better of the two by most people.
I'm guessing op is trying to say the ui is nothing to write home about, quite true, but as for as doing what it's made for, which is to read books, it does just that!