Students can already save a few bucks by opting for a digital version of a textbook over a hardcover, and they can now save even more courtesy of Amazon if they aren't too intent on hanging onto the book after they're done with it. The company has just announced textbook rentals for Kindle, which promises to let students save "up to 80 percent" off the list price of those often pricey textbooks. That discount varies depending on the rental period -- which can be anywhere from 30 to 360 days -- and the option is already available on "tens of thousands" of textbooks from a number of publishers including John Wiley & Sons, Elsevier and Taylor & Fr
