As I understand it with the Nook, all books you purchase are stored online on their servers. If you lose or destroy your Nook you can download all the books again. You can also remove books from your Nook and store them online and download them later when you want too.

As to pricing, from what I have seen with the Nook, (as I stated I bought one for my mom but it hasn't arrived yet) most digital books that are new are $9.99, which is cheaper than the $25-$35 you pay for new hardcover. The benefit for mom is that she can now read any new book because she can use the larger font rather than waiting/hoping for her favorite authors to publish a Large Font edition. From there they descend in price down to FREE. B&N also has a massive library of old classics that run from Free to a couple bucks. I love reading classics like HG Well, Verne, etc.