This is not an anti-Kindle rant. I have purchased (rented?) several Kindle titles myself.
However, YSK that you are only licensing access to the book from Amazon, you don’t own it like a physical book.
There have been cases where Amazon deletes a title from all devices. (Ironically, one version of “1984” was one such title).
https://www.nytimes.com/2009/07/18/technology/companies/18amazon.html
There have also been cases where a customer violated Amazon’s terms of service and lost access to all of their Kindle e-books. Amazon has all the power in this relationship. They can and do change the rules on us lowly peasants from time to time.
Here are the terms of use:
https://www.amazon.com/gp/help/customer/display.html?nodeId=201014950
Note, there are indeed ways to download your books and import them into something like Calibre (and remove the DRM from the books). If you do some web searches (and/or search YouTube) you can probably figure it out.
I‘d recommend the software calibre. Great for managing your ebook library and it can convert epub into amazons azw, mobi or kfx formats (depending on which generation kindle you have). With the right plugin you can even create WordWise data for your kindle-converted ebooks.
You don’t even necessarily need to illegally download the books, as calibre can also handle the DRM of .ebub books you bought from almost any store. Of course, sailing the seven seas is still always an option though.