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 looked in to the whole DRM removal thing. From what I could tell, everything was majorly out of date, required a really old version of Calibre, and didn’t work with newer books.
The DeDrm plugin and the most recent Calibre worked for me just yesterday on a brand new book. Something that’s easy to miss is that you need to put in the serial number of your kindle device and make sure you download the e-book for that same device. Otherwise the plugin won’t be able to decrypt it.
Calibre can be recent - the plug-in was abandoned but forked, but an old physical Kindle is beneficiary. However some books in the store are no longer available for those lately.