Welcome to the Metadata Millennium: A Complete Overview of What Metadata Can Do for Publishers
The most familiar of these identifiers, of course, is the ISBN (International Standard Book Number). But despite what its name implies, an ISBN doesn't identify "the book" as the publisher thinks of it. Instead, it is a product identifier, designed for the supply chain to enable the precise identification of a specific product. Contrary to what publishers often think, there is no such thing as an "eISBN": each distinct ebook version -- EPUB, Mobi, PDF, etc. -- needs a distinct ISBN, just as the hardcover, paperback, and audiobook do, because if somebody wants to buy the EPUB they don't want to get the PDF instead.