An ISBN is used to uniquely identify a title and format. Just as separate ISBNs are required for a hardcover and soft cover (perfect), digital versions of a title require a unique ISBN. Furthermore, the ISBN guidelines specify unique ISBN assignments are needed for varying ebook format (.mobi , .epub, pdf).

Quick Facts:

• The ISBN used on a printed book CANNOT be used on the ebook.
• Each digital version requires an separate ISBN.
• Substantial changes in content require a new ISBN. Note: A new ISBN is NOT required for a change in price, but a new barcode may be required for printed versions with pricing printed in the barcode symbol.
• Some ebook publishing services allow publishers to use an ISBN assignment from their own assigned block.  Although this is completely acceptable, the publisher will not be able to use the same ISBN assigned by the ebook publishing company in other marketplaces.