Amazon just issued a press release announcing the opening of their Kindle Textbooks store. Titles will be available to rent for periods from 30 days to 360 days, and students can increase the rental period in increments as small as one day, or purchase (license) the book outright at any point.
See: http://www.teleread.com/amazon/amazon-announces-textbook-rentals-for-the-kindle-platform/
Textbook Rentals Come to the Kindle: Probably Not a Money-Saver
Textbook Rentals Come to the Kindle: Probably Not a Money-Saver
http://www.nytimes.com/external/readwriteweb/2011/07/18/18readwriteweb-textbook-rentals-come-to-the-kindle-probabl-85881.html?ref=technology