You can only store the books you buy from Amazon on Amazon. They also will only keep the books as long as they sell that book. If they discontinue the book you won't get it back.
They don't really store "your" book. They just keep track of the books you've purchased and downloaded. If you want to reload a book they will reload it from their inventory.
Any books or other media you download from any other source will not be available at Amazon.