Prime content isn't free, it's paid for (sometimes poor payment to publisher/author) by your subscription.
Check out 1000s of real versions of all the classic stories rather than sterilised saccharine Disney ones in the supermarket.
www.gutenberg.org
Really free because copyright has expired. Many of us originally bought eink ereaders purely to more easily read content from Gutenberg.
Calibre is also ideal to fix any silliness in the ebooks. I convert them with automatic removal of paragraph spacing (put 1.4 em indent), "beautify quotes" to “Proper Quotes” and I also put line-height in "remove styles" which is good for bought books.
I download all Amazon books "for transfer via USB". Content and Devices.
I never turn on WiFi on Kindles or Kobo and none of my Grandchildren have their local WiFi passwords. All sideload.