The potential to spread knowledge using ebooks is just so incredible and exciting. Instead of a poor child having limited access to books, a single e-reader device could give them HUNDREDS of books for free from the public domain. There's nothing better than giving a child the opportunity to learn in such a way.
Such a device would need to be rugged (as you mentioned), cheap and ideally be able to handle software upgrades. It wouldn't even have wifi or anything, just a simple charger and a usb port. A very simple device so it can be sold for $50.