Kindle Owners' Lending Library
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From Amazon's Website:
The Kindle Owners' Lending Library allows eligible U. S. Amazon Prime members who own Kindle devices to choose from thousands of books to borrow for free -- including more than 100 current and former New York Times Bestsellers -- as frequently as a book a month, with no due dates.
Basically what Amazon is saying, is that Prime Members get up to 12 free book rentals a year. Make sure you make a good selection on what books you rent. I read some places online where someone rented a children's book, and now they have to wait till next month to rent another book.
Make no mistake, there are good choices in Amazon's Prime library, just remember that what ever you select, that you have to wait until next month to select another book.