Do you need a separate ISBN for your ebook?
According to PublishingBasics.com, the answer is yes. With an asterisk.
This is probably one of the most asked; least understood questions out there in the self-publishing world today. Technically you not only need an ISBN but you need a different ISBN for each format your ebook is being offered. If you are offering your book as a mobi, epub and a PDF, you need three different ISBNs. That’s technically. In reality, if you are only making your eBook available on Amazon, you don’t need any. Confused? Read the entire piece.
An ISBN is merely an identifier for your book that allows it to be sorted an classified by people/organizations who may deal with it in various capacities. For example, it helps librarians to archive it properly. It helps distributors store and ship it in an organized manner. And it helps retailers order, store and place in in the correct places in their stores.
Book titles can be veery similar, but the numbers make everything much easier to sort and organize. And the numbers can also be used to identify new editions, formats and other iterations.
So a new ISBN is a no-brainer for a format change — such as, say, between a print edition and a digital edition.
If you’re dealing with…
If you’re selling e-books directly to end-users, I really don’t see the necessity to assign a separate ISBN to your digital edition.
The PublishingBasics piece has an excellent Q&A that uses a representative from RR Bowker as its source. It’s an excellent place for further information on the topic.