Download something called Hexchat and go to Undernet, channel bookz. This is a bit technical as you have to use IRC jargon to search for and download books.
You can find most book there in epub format (occasionally mobi too). Use the Calibre application to convert
epub->mobi.
You can then plug in a kindle to a pc (or mac), using a USB cable, and just drag the book into the
Documents folder. The book will then magically appear on the kindle menu when you hit the home button (sometimes you may have to scroll down to find it).
You can also email books to <username>@kindle.com (replace <username> with yours) if you have an amazon account but this takes a bit of setting up. You should be able to find out how fairly easily on the Internet though.
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Another option is the Gutenberg website
https://www.gutenberg.org/. There are 1000's of books there. txt, mobi, epub, pdf etc....
e.g. The Scarlet Plague, Jack London
The Gap in the Curtain, John Buchan
The Mercy of Allah, Hilaire Belloc
Complete Works of Shakespeare
Grimms Fairy tales.