You've probably heard about Amazon Kindle, right? I had this idea years ago. Probably lots of people did. There might've been similar products that came out in the past without much notice because the technology just wasn't good enough to replace good ol' books yet.
Now, the screen is like paper, you can download books and stuff wirelessly basically anywhere (not just in WiFi hotspots), and the whole thing's really thin and light... like a little book or something. Oh yeah, and the books are totally cheap since there aren't any printing or storage costs. If you haven't seen it yet, check it out. It's pretty slick.