This question already has an answer here:
I have a Kindle ebook reader. How can I liberate ebooks that are stored on it? (transfer to my PC and make them readable on any device).
Also, I tried copying my ebooks to Kindle directly, but they don't get recognized in any format I tried. I send them to my email address @kindle.com and they appear on my Kindle alright, but they get converted to azw3 format. Is there a better way to transfer ebooks to Kindle, e.g. just make it read mobi files?
I use Linux, so I can't use Apprentice Alf because apparently it is Windows and Mac-only (even though Calibre works on Linux)
Recap: I'm concerned about freedom of both Amazon and non-Amazon content:
I would like my ebooks bought from Amazon to be readable on other devices (the most pressing issue)
I would like to be able to get my ebooks (not bought from Amazon) back from Kindle, if necessary.