Well our little experiment worked until I added another book shelf and book. I decided to test it by removing books from B&N memory and adding them to the SD card instead, this made it forget the one book I had saved on a shelf to the SD card. So I'm going to say its still a no go.
Although if your rooting, 5 gigs is a lot of memory to hold books. You can use the SD card to hold music, Apps, etc so it may not be bad. I realized after I had loaded on about 200 of my ebooks I've bought over the years that I haven't even put a dent in the 5 gigs it has free.
