The Sony 505 has explandable memory and an extremely long battery life (usually two weeks, 3,000 page turns). If you get something with wifi or touch screen it'll shorten the battery life. You can buy from several different websites, as long as they have a Sony readable format (LRF is my favorite).
You can also get them on sale brand new, you just have to keep your eyes open. Circuit City had them on sale, but I don't know if they have any left.
I super duper love my 505
I've always been a bookworm.