Good question. Why, indeed?
The International ISBN Agency says that every format of a book that you sell must have its own ISBN assigned to it. Most people understand that, if you have a paperback and hardcover edition of your book, you need two separate ISBN numbers—one for each edition. But, when it comes to ebooks, lines get blurred.
There’s no such thing as one ISBN number for all ebook formats.
You’re probably thinking, “I have a paperback, a hardcover, and an ebook. That’s three! Why do I need more?”
I say, “Nuh-huh. That’s not three. How many ebook formats do you have?”
You see, an ebook is not a single format, at least not anymore. Ebook formats are epub, mobi, pdf, and so on. Ebook became an umbrella term for all digital book formats. Plural. They are as different as hardcover is from paperback, and each of them needs its own ISBN.
So, to get back to why it’s not three…
You don’t have “a paperback, a hardcover, and an ebook.”
You have “a paperback, a hardcover, and __xyz__ formats of an ebook.”
The more formats of an ebook you have, the more ISBNs you have to get.
Of course, if you publish only an epub edition, then you do have one ebook, and you don’t need more ISBNs. But, let’s say you published an epub on Barnes&Noble, and a mobi on Amazon Kindle. That’s two editions of your ebook. They each require their own ISBN. Additionally, if you sell a pdf ebook edition on your website or blog, that’s a third format, and it should have its own ISBN.
In the end, you have “a paperback, a hardcover, and 3 formats of an ebook (epub, mobi, and pdf).” That’s a total of five ISBNs.
But, it’s not as grim as it sounds…
Some publishing platforms don’t require that the files you give them for conversion have an ISBN.
For instance, Amazon assigns their ASIN number to your ebook, and ISBN is optional. Smashwords offers free ISBN (if you don’t have your own) that you can assign to your ebook, so you don’t have to buy one.
In other words, if you intend to publish only on Amazon and Smash, you don’t have to buy your own ISBNs for ebook editions. However, the best practice would be to have one ISBN for each edition of your ebook (one for epub, one for mobi, one for pdf…).
Special notice on Smashwords ISBN: While it’s not necessary to place ISBN inside your ebook, it is mandatory to attach it to your book via Smashwords’ ISBN Manager feature on your Smashwords dashboard. If you don’t have an ISBN, Smashwords offers one for free. That free ISBN is for epub edition of your book that Smashwords will send to their network of retailers, and it is not for other formats of your ebook that they will generate (such as pdf, mobi, and so on). That means your other ebook formats generated by Smash conversion don’t have ISBN—only Smash-generated epub does.
Thanks for reading!