Those of us who use ereaders for book reading however use it for that specific purpose.
Ereaders use electronic ink (E Ink) and not a back-lit screen. This is much better for the eyes, and it feels much more like reading a real book. They also have very long battery life since they don't use any battery while resting on a page, only while changing pages.
The drawback is that you need a light source to be able to read on an ereader.
The kindle is the most popular ereader out there now, and the basic Kindle has reduced the price to only $79 last time I checked.
There are other ereaders out there that may be even cheaper, and that still use the electronic ink technology. I bought one for $70, the BeBook club, about a year ago; which is all I need to read epub, txt, pdf, and other ebook formats. It can also zoom in and out and make the letter bigger or smaller like all ereaders can. But doesn't have text-to-speech, dictionary, and wifi like the kindle does. That's pretty much all I need for book reading and I'm very happy with it.
Kindle Fireis Amazon's way of competing with tablets like the iPad. It's about half as cheap and almost as powerful, but it lacks access to the apple marketplace for apps, or the android marketplace (at this time), they have their own application marketplace with games, movies, etc. It's good enough for browsing the web, writing and so forth, as well as watching movies. Lots of movies available on the Amazon marketplace.
Kindle Fire Vs. iPad 2 vs Nook - Specs and features
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