How can I return a borrowed Kindle book from a lender?
You can do this for Nookbooks too... there is a return link to the right of the book. After returning it, you can then delete the title or buy the title, as you wish. Or one can opt to just let it expire after the 14 days are up. Me, I like to return early if I finish the read early. I just feel that it is polite.
I read most books quickly, so it's not at all unusual for me to return a book only a few days after I got the lend. A recent 700 page read only took me two days to finish, because I was sick at home and just engrossed by the book.
You do not have to do anything because the book is automatically returned to the lender after the loan expires. It is possible to return a book early by going into the Manage Your Kindle Page and deleting the book. This will immediately return the reading rights to the lender.