When I researched this a while ago, the consensus was Bluefire Reader as the best of the two options (that & Stanza), but I see updates have removed that option. Fortunately I got it while it worked, but that doesn't help you!
Stanza still says it will work, but 3.2 just crashes upon opening for me, apparently a common problem with all iOS versions. Amazon bought & killed it.
You might try DB element's Reader lite - http://dbelement.com/apps/reader & https://itunes.apple.com/us/app/reader-lite-powerful-ebook/id346753851?mt=8 - it just does plain text and PDFs (no epub), but it's something. The app gave me an error and sent me to the browser, but I was able to upload a book to the DBelement site and view it through the browser so that should work - it says offline on the web page I gave above. The web site says 2 books and then you have to pay, but I don't know if that's 2 total or 2 at a time.