Several people have reported good results with backup extractor (search the forum and google). One you have the book on your computer, you will have to strip the drm in order to read it on any other device or reader. You'll have to search the forum for old instructions on how to remove drm as it is against forum policy to provide details.
BTW, you don't "buy" or "own" Kindle books, you lease them. There are many posts and arguments on the subject all over the forums.
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