Two options:
1. Root and install Amazon's Kindle app. Rooting should be relatively safe, but keep in mind that you're intentionally doing something that the reader wasn't initially designed to do. Things may break. But there's also a fairly large community of devs who are dedicated to this stuff and can help you out if you run into problems.
2. Remove DRM and convert. Neither step is difficult, but I will not talk about the former (DRM removal) here except to say, "Apprentice Alf is your friend". The latter is pretty trivial with
Calibre. Mobi (Kindle's ebook format) converts to epub very nicely and Calibre is a mature product that has been doing it for a while, and in fact is often used by publishers and others to convert between various formats (gasp!).
I do option #2, simply because it's easy enough to do and I have no other reason to root my Nook STR.