One of the reasons that there is limited selection of e-textbooks is the difficulty with the electronic format. Tables, charts and graphs must be specifically adapted for the e-reader (and I think that Kindle and Nook use different formats) or they won't display properly. I have a 2nd generation Kindle and while it is a fantastic device for text, it's a real challenge for non-text info. I know this firsthand because I made the mistake of downloading "Freakonomics" for my Kindle and it was almost impossible to read because of the graphic material in the book.
The format issue is not an insoluble problem but until it's resolved (and it very well may with the release of the 3rd gen Kindles) I would not want to use a Kindle as textbook.