Well first of you need to decide if you want the ability to read abosolutely anything even obscure titles, if so go with the non-tablet that Sylvie has called the Kindle. Let's face it amazon has everything.
The Barns and Nobles Simple Touch is technically a tablet not an electronic book reader and where that really shines is in the interface.
Kobo Touch is also a tablet and its, from what I hear the simplest thing out there.
An ipad is definetly a tablet it its very good at what it does, troubble is that its proprietary meaning you are forced to buy the apps that Apple says you can have with no competition. For some that may be ok you need to decide for yourself.
For more information I recommend
http://www.pcmag.com/category2/0,2806,2358246,00.asp and
http://www.pcworld.com/products/tablets.html