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Sounds like you are talking about the old DRM problem.
Sony, Kindle, B&N use different DRM implementations.
That is if you buy an DRM ePub book from Sony Bookstore you can't read it on your Nook unless you remove DRM from the book.
Kindle books can't be read without Kindle (software) unless you remove DRM from the books.
Unfortunately most stores are selling DRM protected books which is a problem when you try to change devices. Best to buy books without DRM. That way you can copy+read them to any device which supports the format.
BTW when you buy a kindle-book from Amazon you own the book. Which doesn't mean you can do with it whatever you want.
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