From stackexchange
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Amazon's Kindle for PC reader saves a copy of your books to your hard drive, once you download them. Note that you have to download them from the cloud; new books aren't downloaded to a device unless (1) you told Amazon to send that book to that device or (2) you downloaded the book from the app on that device.
By default, the books are saved in this folder:
C:\Users\YOURUSERNAME\AppData\Local\Amazon\Kindle\application\content
Replace YOURUSERNAME with your Windows username.
Typically, you will have two files per book: a .apnx and a .azw. Backup both of them. You may want to consider an automated backup service to backup these and your other files.