The return process for library e-books and e-audiobooks is usually simple. You don't need to do it. Once your loan expires, your access to the book expires so other individuals can check it out.
You may want to return items early so others can have access to them sooner. Here's how to return a library book checked out to a Kindle device
Return a Library Book
You'll receive a courtesy reminder e-mail from Amazon three days before the book is due and another e-mail when the loan period has ended. To return a library book:
1. Go to Manage Your Content and Devices.
2. From Your Content, select the Actions button next to the book that you want to return.
3. Select Return This Book, and then select Yes in the pop-up window.
You will no longer have access to the book from your device after it's been returned. However, you can always access your saved notes and highlights through Manage Your Content and Devices even after the book has been returned. If you check out the book again, or purchase it from Amazon, your notes and highlights will appear in the book.