I know that a different ISBN is needed for the same title in hardcover, paperback, and e-book. However, I am confused about e-books. Amazon's web site says you don't need an ISBN to publish a book to Kindle through them. Is this because you don't need an ISBN, or is it because Amazon will obtain an ISBN and assign it to your title? Suppose you publish an e-book through Kindle (which I have done), and used an ISBN you obtained from Bowker. If I now want to issue the same book as a Nook title through Barnes & Noble, does it use the same ISBN that was used on Amazon, or do Kindle and Nook versions of the same title (and same edition) share the same ISBN number?