Each edition of a book, including each ebook edition (each format is its own edition), that is to be sold through retailers, needs a unique ISBN. An ISBN is the International Standard Book Number consisting of 13 digits (after 2007).
Book distributors and retailers rely on ISBNs to identify and track books. Not having an ISBN can therefore be a barrier to distribution. Amazon has its own optional “ASIN” alternative, but this is of no use with other distributors and retailers. ISBNs remain the most widely used system for books.
I recommend that the ISBNs for all available editions be listed on the publisher page. This is an example of how you can list them:
ISBN: 000-0-0000000-0-0 (Hadcover edition)
ISBN: 000-0-0000000-0-0 (Paperback edition)
ISBN: 000-0-0000000-0-0 (Audiobook edition)
ISBN: 000-0-0000000-0-0 ebook (ePub)
ISBN: 000-0-0000000-0-0 ebook (Kindle/Mobi)
ISBN: 000-0-0000000-0-0 ebook (ePDF)
eBooks conversion does not automatically convert the ISBN in the meta data.
Having this list on your publisher page allows bookstores, librarians, and readers to see all the available options, which can help increase sales. For example, readers can now see that a hardback is available if they prefer hardbacks for gifts or what ebook edition to order so they can be sure they are getting a format that works on their preferred e-reader devices.
If you are an independent publisher, it is important to understand that the third set of numbers in the ISBN represent the registrant or publisher.
That is, when a bookseller, buyer, or librarian uses the number to look up your book, those numbers need to identify you, the publisher. If you buy numbers secondhand (available through companies that buy them in bulk and offer them at a discount), the number will be registered to someone else whose brand may not be helpful to your company.