If you want a device primarily for reading mostly text books, with not a great deal of extras, either the Kindle or one of the Nooks are what I'd recommend. I'm personally partial to the Kindle. The major pluses for the Kindle 6" (and probably some of the other dedicated e-readers but my most experience is with the Kindle) is the Text To Speech, it is by no means on the par of an actual audio book but since audio books are a bit like large print, hard to find and usually costly, it's a great alternative. One I use a great deal. Then the e-ink, the Kindle I'm able to forget it's a device and not a book most of the time. It feels close to holding a book. In fact, for the first few months I had it, I'd find when reading a paper or hard back book, I was constantly catching myself trying to look up a word, then realizing that I was not reading on the Kindle! The Kindle 6" costs any where from $114 for one that is WiFi only and has advertisements as the screen savers, $139 for the WiFi only one with the standard screen savers and $164 for the 3G and WiFi with advertisements and $189 for the 3G and WiFi with the standard screen savers. I personally would spring for one of the ones that has both WiFi and 3G, it's not that big an amount difference and it's a one time payment to double or triple the places you can access the Internet and your archive.
If you want to read things with a lot of graphics, use huge font sizes all of the time and a lot of extras, an iPad is what I'd recommend. One of the first generation ones is actually less than the Kindle 9" (depending on the amount of memory and whether it's WiFi or 3G) and is a much greater bargain. I think a reconditioned one is going for around $350, for WiFi only and about 16 GB memory. As a reading device, the drawbacks are the backlight, though at least in the Kindle app, you can change the background to black with white text, which is much better for night reading and you can also change it to sepia colors, which I find easier on the eyes in daylight reading. No Text to Speech and you never forget it's a device. However it does a lot of other things, surf the web, listen to music and my personal favorite, watch movies, either streaming from Netflix and other places or from internal memory, and a great many other things. On the iPads, WiFi only is probably what I'd recommend, the 3G capability costs somewhere around $100 extra and then you have to do a monthly plan. They seemed to be a major PITA and to cost quite a bit per month when I looked into it. That was about six months ago though, so that might have changed. Just my opinions for what it's worth, anyway.