Chegg

BookRenter offers a little more flexibility in their rental periods than Chegg, with five different rental lengths: 30 days, 45 days, 60 days, Quarter (90 days), and Semester (125 days).

If you want to keep your books permanently, your rental price will go towards the sale price of the book..

Source: http://www.digitaltrends.com/buying-guides/where-to-rent-cheap-textbooks-online-for-college-chegg/?news=123