You may fondly remember standing in line at your college bookstore to trade pricey stacks of pressed tree into measly amounts of hard currency, but Amazon's determined to tear that paradigm down. Today, it's launching Kindle Textbook Rentals, which promises to save you time and money by making the entire transaction digital. Rather than buy this expensive chemistry hardcover — or even its e-readable Kindle alternative — Amazon will let you borrow it for your portable devices for up to a year at a time, paying a smaller chunk (in this case, about 40 percent for 60-day rental, or about 80 percent for a year) of the regular purchase price. What's more, you can apply your full rental fee towards a longer rental or even towards a purchase if introductory chemistry turns out to be handier than you thought, making Kindle rentals a no-brainer if you were already thinking of buying a Kindle copy. Of course, you'll still have to reconcile the idea that you'll be renting bits and bytes from Amazon's infinite supply at what's still a fairly sizable cost, but if you're not used to the idea of being charged dearly for textbooks, you've probably never been to the college bookstore.