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.
IMPORTANT NOTE: Software that converts a Mobi (Kindle) file into an ePub file (or the other way around), does not automatically change the ISBN meta data during the process. Both books may end up with the same ISBN embedded in the meta data. The change can be made by editing two of the html files that are inside the converted edition.
The current Bowker price (January, 2016) for one ISBN is $125 and for ten ISBNs is $295.
eBooks have no physical presence so a barcode is not required.
The ISBN is a set of numbers and this is separate from a barcode. A barcode is a representation of the ISBN numbers. eBooks have no physical presence so a barcode is not required. Barcodes are used by publishers for inventory control of the physical books and used by retailers for the same purposes as well as identifying the price of the book at the point of sale if the barcode includes a price. The barcode is generated from the ISBN and can include the retail price of the book. If you are working with a professional book designer, he or she can usually generate a barcode for your print editions. It is good practice to create the barcode late in the process because the price of the book may change based on evolving considerations such as final page count, printing costs, etc.