This is a really good question. The tablets have advanced and improved so much just in the last 3-4 years.
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My father gave me a Simple Touch Nook which was a plain reader but so easy on the eyes. I learned to use BookBub, as well As Overdrive from the library. I now have the Nook HD and like it but the smaller Simple a Touch was easier to hold. You really need to try them and imagine yourself in bed reading or on a beach with bright sun. The Kindle works with BookBub and Overdrive as well. I read so much more now thanks to these e- readers. They are great for eye problems too as you can enlarge the Font size or change the Font. Have fun with your search. The e readers have "e-ink" which makes the print easy on the eyes.