I've looked into all these, and I have to say that if all you want the device for is to read books, then just get a book reader, ie. Kindle. If you want an all-in-one solution, use something like an iPad.
As already mentioned... your eyes get tired much faster reading on an electronic device vs. reading a book. If you want to store hundreds of books, then an ereader is the go... they have their pro's and con's.
I have an iPad, I have two apps on it, Kindle, for downloading and reading Kindle publications, and Goodreader, which is a current highly downloaded reader app for pretty much most standard file types. It also allows you to draw on, highlight, mark, tag, etc etc etc.... all the good things you can't do with Kindle and most ereaders, though you obviously need a PDF or other document format to use Goodreader. Still... hurts your eyes faster than reading a paper book. The Kindle app is the same as reading on a Kindle only device, except you get the benefits of the iPad features, zoom, enhance, etc.
I can sit there and read a couple hundred pages in a book consistently, I can't using an ereader, as my eyes become sore after 20+ pages.