Have you seen some of the other ebook apps listed in the
The Big List of Android eBook Reader Apps thread?
You can definitely sideload epub's with the Nook app by putting your ebooks in the 'MyDocuments' folder within the 'Nook' folder on the sd card. It unfortunately requires a fixed location, which may differ from your usual storage place used for your ebooks, and therefore causing duplicates to be stored.
I haven't used many of the Android ebook apps, but do like coolreader; it's very customizable and is even open-sourced...
Explore them at your leisure!