If it's Amazon, they're gonna remotely delete your Orwell. It seems that Kindle runs a modified version of Ubuntu which is cool-ish, but it's difficult to swap the stock Amazon firmware. Also, Amazon's record on home-spy-devices, like Echo and Ring, is piss poor.

If you find some ebook reader that doesn't connect to the internet and phone home to receive updates then it could be considered more like a microwave oven and I wouldn't worry too much. But if it's running Linux and connected to wifi, then that's a very capable backdoored device to introduce into your home.

Edit: As @moonlune pointed out, Amazon runs ads on (some) Kindles. That's another issue with the OS being proprietary.