If you have an ebook in mobi or epub or azw3 format that isn't DRM protected, Calibre has the ability to convert it to whatever Kindles like out of the box.

If you have DRM encumbered files obviously you're going to need to do something else, first, but why did you buy a book with DRM? Seems unwise to begin with lol

Can you imagine buying a dead tree book and have the publisher come up and say "no no no, you can't sell that when you're done!"

DRM'd ebooks. What a world.