Bluefire reader 1.7 works fine even on my ancient first gen iPhone. Unfortunately, it's no longer available for download in the AppStore.
However, Blufire Inc. has licensed their code as White Label Apps to
a number of companies and not all of them have updated their code base. One of them is libri.de whose older
eBookS Reader app is still available in the iTunes store. It lists OS 3.2 as the system requirement. (Since the screen shots are in German, it might have not been localized into English, though.)
If that app doesn't work for you, try searching for older apps of one of the other licensees.