I can just see some readers hanging off the shelves by a chain in a bookstore, like earphones in a music shop, where you could pop in the SD card to have a browse... I think it's a good idea, if the reader can handle both files and cards you'd always be able to load plenty of other files (from Project Gutenberg, for example) on to your device, and as you say, carry several book cards with you with little effort. There would have to be a software available to make backup copies of your cards (using a licence key to enable copy?), or some way to protect yourself from loss of the same, in the same way you can photocopy your book, just in case your dog eats it (it's happened to me, sigh). I'd be worried the cost of the card would make the books expensive, though. How likely is that?