If you are not seeing the option to return the book on the 'Manage Your Kindle' section that is most likely because it is a book published by Penguin and these are not able to be returned early. Did you have to transfer the book via USB? That is usually the sign that it is a Penguin book. If that is not the case can you please email back with your library card number and the book title please? Then we can look further into it.