Amazon announced this week that they are offering textbook rentals
for the Kindle. The rental period lasts from 30 days to one year, and the price varies by rental time and title. The rental service is available via the Kindle app on many different devices, so you can use this service even if you don't own Kindle. That last point main be a real advantage for usage as textbooks will look much better using the Kindle app for the iPad, PC or Mac.
Wired has a nice review