That type of content you can try to find through google, or through a library site, one that may work for you is https://www.free-ebooks.net/
Then you'll need to download the book files to your computer,
plug the kindle to your computer with a usb cable, and then look in your Finder, you'll see (along with your hard drive, cd drive, etc) the kindle appear as a removable memory drive. You'll then drag/drop or copy/paste the book files to the DOCUMENTS folder on your kindle.
After you disconnect it, then they'll appear on the kindle home page.
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