Unless one is planning on using an ebook reader as a substitute mp3 player or storing loads of PDF files as well, an SD card shouldn't be much of a concern to most people. The native memory on any of the ebook readers can hold hundreds of books (ebook files are amazingly small - my Collected Works of Mark Twain is 8mb). And there's nothing to prevent storing thousands more backed up to other media like computer hard drives, CDs, etc. In addition, Amazon stores any ebooks purchased from them on their site, and I assume that B&N does the same thing.