I found this article here
Spoiler: The best options, which are the only ones that are discussed are Kindle, Nookcolor, iPad and the Galaxy Tab.
Apparently Nookcolor is the strongest rival of the Kindle, and the author might not be aware of the fact that there is a Kindle app for the iPad:
Students will find fewer textbooks available in Apple Inc.'s iBooks than in Amazon's Kindle store.
(the above statement is listed as one of the cons of the iPad)