As a publisher, you’ll obtain a block of ISBNs specific to your publishing name and assign a unique ISBN to each edition of every book you publish. Each edition of every book? Let’s clarify what that means.

Assigning a unique ISBN to each new print edition
Let’s say your first book is a novel called Raven and it’s a hardcover book. That means your first ISBN number will be assigned like this:

ISBN 978-0-1234567-0–0 / Raven / Hardcover Edition

The second-last number (in red) is 0, the first digit in your block of ISBNs. This number is always assigned to the first edition (in this case the hardcover edition) of your first book. The last number (in blue) is automatically calculated by an algorithm, and it’s called the “check digit”. (Sometimes the check digit can be an X, so don’t worry if you get an X as the check digit in one of your ISBNs.)

Let’s say you next release a softcover edition of your book. You’ll assign the next ISBN in your block of numbers to your softcover edition, like this:

ISBN 978-0-1234567-1–7 / Raven / Softcover Edition

The second-last number (in red) is 1, the next digit in your block of ISBNs. And the last number (in blue) is the check digit. So far so good, right?

Assigning ISBNs to eBooks
There are three main formats for eBooks: Mobi (for Kindle), ePub (for other eReaders such as Nook, Kobo, iBooks), and PDF. And this is where assigning ISBNs gets a bit trickier, as it depends on how you intend to sell your eBooks.

Selling eBooks yourself through different eBook retailers
If you’ll be selling your eBooks yourself through different eBook retailers, you’ll need to assign one ISBN for each eBook format. For example, if you sell your Mobi format at Amazon’s Kindle Direct Publishing and your ePub format at Barnes and Noble, you’ll assign one ISBN for each of those formats, like this:

ISBN 978-0-1234567-2-4 / Raven / Digital Edition (Kindle)
ISBN 978-0-1234567-3-1 / Raven / Digital Edition (ePub)

Selling eBooks through an eBook service
Several companies will act as a sales channel for all your digital editions. Bookbaby, Smashwords, and Vook are examples of these services. If you’re using one sales channel for all your digital editions, then you’ll need to assign one ISBN for that sales channel, like this:

ISBN 978-0-1234567-2-4 / Raven / Digital Editions (Bookbaby)
or
ISBN 978-0-1234567-2-4 / Raven / Digital Editions (Smashwords)

Can your eBook be published without an ISBN?
Yes. If you are only publishing a Kindle edition through Amazon’s Kindle Direct Publishing, you can use their internal ASIN tracking number to track your sales instead of an ISBN. Or, if you are only selling eBooks directly from your own website, you can choose not to assign ISBNs to them.

Keep in mind that publishing eBook editions using your own ISBNs means that you’ll be listed as the publisher in the appropriate Books in Print database, and that may help readers search for your eBook online.

For more information about assigning ISBNs to eBooks, visit Bowker’s eBook FAQs and International ISBN Agency’s FAQs.

How do I tell if my updated book is a reprint or a Second Edition requiring a new ISBN?
If you are ordering another print run of your book with no substantial changes (just a few typos, for example), then you don’t need to assign a new ISBN. If you’ve changed the cover but not the text, you can continue to use the same ISBN.

If you’ve changed the content of your book or added new material (another chapter, preface, appendix, or other content, for example), then you’ll need to assign a new ISBN. A new edition is considered a different product and therefore gets its own ISBN.

I hope this answers your questions about assigning ISBNs to different editions of your books!