Kindle Paperwhite for just reading books. I love my Paperwhite. Amazon has one less-expensive eInk screen model (the unlit model) is not easy on the eyes. The Paperwhite is their lowest price lit model. Paperwhite has adjustable frontlighting so it can be ready comfortably in all lighting from full sun to full dark. It has no color, no audio, no video. Note - Amazon has some eInk models that are more expensive than Paperwhite but I doubt they add anything you need. I know they don't add anything I need.
I recommend this Paperwhite case for good protection and auto-on when you open the cover. The cover can fold all the way back and the case provides decent protection to all sides and corners. I think the solid colors fit a bit better and immediately but I recently got a patterned one for my inlaw and I just needed to flex the cover spine back and forth a bit to loosen it up.
For downloading ebooks (and for registering it) you need a Wifi network (wireless internet). Alternately, ebooks you purchase at Amazon can be downloaded to a computer and loaded to the Kindle over USB but it really is a lot easier if you have Wifi.