answer:
An ISBN is a unique number that identifies your book. The number is registered with the international ISBN agency and theoretically it helps customers and sales outlets discover your book and differentiate it from other books. The ISBN agency requires a different ISBN for each version of your book. If you have a printed hard back book, that requires a number, if the same book is reprinted in paperback, that requires another ISBN number, likewise a digital copy of the same book requires yet another number.
