(This is modified text from The Global Indie Author.)
The ISBN System
ISBN stands for International Standard Book Number. The rules and regulations governing the use and distribution of ISBNs are determined by the International ISBN Agency, based in London. The international agency allots ISBNs to national agencies, who in turn allot them to their publishers; thus, a publisher cannot acquire an ISBN from a foreign national agency: a British publisher, for example, cannot buy an ISBN from the American ISBN agency and vice versa. Publishers then assign their ISBNs to their books. ISBNs are thus owned by the publisher, not the author. An author can, of course, also be the publisher, but they own the ISBN in their capacity as publisher.
Not Mandatory, Just Practical
ISBNs are not legally required to publish; however, an ISBN is the primary metadata by which retailers, wholesalers, distributors, libraries, and national cataloguing agencies identify a work and its format. If you do not assign an ISBN to your print book it cannot be distributed through traditional means, and it cannot be registered in the various databases from which book buyers make their purchasing selections.
A single title may also be published by different publishers in various formats, or “editions”: the hardcover, softcover, mass market paperback, audiobook, and so on. Each edition is assigned its own ISBN, which is unique to that publisher/format combination. The system ensures that when a customer requests the hardcover of a title they do not get the paperback instead. A title might also be published by different publishers in different jurisdictions; again, each publisher assigns their own ISBN to the title. Since the ISBN is unique to the publisher, the market can keep track of who publishes what and where.
ISBNs for eBooks
With ebooks, the same principle applies: different ISBNs must be applied to a title’s different digital formats — Kindle, ePub, PDF, and so on — to ensure that the consumer does not order the ePub when they need the PDF. If multiple versions of the same file format are published with different digital rights management (DRM) software applied, each file should be assigned its own ISBN. And if the same file using the same DRM but where different versions apply different levels of security — one PDF might allow printing, for example, while the other PDF of the same title may not — different ISBNs should also be applied to each file. And if you were to publish a static PDF and later create an interactive PDF version of your book, that interactive PDF would also need a new ISBN.
However, most ePubs are distributed via aggregators, and the indie author is not given the option to upload different ePubs with different ISBNs for distribution. The indie author really has no choice here but to distribute a single ePub with a single ISBN.
The Kindle format is proprietary to Amazon and is not distributed outside of Amazon, so you can publish to Kindle without an ISBN; Amazon applies an internal cataloguing number, an ASIN, to Kindle books. This does not prevent you from applying an ISBN to a Kindle title, however, and if in time Amazon makes the Kindle format available outside its walls your ISBN may prove useful.
Barnes & Noble’s PubIt!—the indie wing of Nook—also applies an internal cataloguing number so an ISBN is optional for B&N; this, however, is only available to U.S.-based authors.
Kobo’s new indie publishing wing, Writing Life, will assign its own cataloguing number for books restricted it the Kobo site; therefore an ISBN is not required. However, if you wish for your Kobo book to be distributed to the additional 30 countries accessible through Kobo’s retail partners, an ISBN is required. And if you elect instead to sell through Kobo via an aggregator, an ISBN is mandatory.
Apple and Sony and other online retailers will not carry ebooks without an ISBN, so you cannot distribute to these retailers without one.
When Does a Book Need a New ISBN?
Minor changes to a manuscript do not require that a new ISBN be assigned. Typos and such can be fixed at any time without concern. You can also add new marketing pages without issue, or modify the cover to advertise an award, for example, or to add a new endorsement. It is only when the manuscript itself undergoes material changes—for example, if a foreword is added or new illustrations are created—or a whole new cover is put on the book, that you need to assign a new ISBN (and the new edition must be indicated on the copyright page).
ISBNs and Copyright
ISBN designation does not confer or register copyright; ISBN and copyright have no bearing on each other whatsoever.