Where do you read the most? Outdoors? In bed?
When you read books, do you prefer large print?
Do you wear eyeglasses? Contact lenses?
How much are you willing to invest in a device?
Would you like to be able to mark them up with notes?
Here are a couple of overviews that might be helpful -
http://www.theguardian.com/technology/askjack/2013/apr/04/best-tablet-for-reading-kindle-ipad
https://www.youtube.com/watch?v=a5TGHnldsKg