I am using Overdrive Media Console on my rooted Nook. First, after I downloaded it from the Market, I activated it (from my PC using Windows Vista Home Premium 64-bit) with Adobe Digital Editions.
Overdrive books are in Adobe Epub format.
I then have download books from my local library - for example I am currently reading "The Spy" by Clive Cussler. I D/L the book to my PC and throw it in my Dropbox - when I open the book in Dropbox on the Nook Color, it automatically asks if I want to read it in the Overdrive Reader (i.e., the Overdrive Media Console) or in the BN reader - I choose Overdrive so I can read in landscape.
I suppose you could also use Dolphin browser on the Nook Color to directly D/L books (or to Dropbox) but I have not tried this yet - the way I'm doing it works well for me and is simple.
This also works with Overdrive audio books so you can D/L them to the Nook also.