1. A print book ISBN cannot be reused for the eBook.
2. It is recommended, but not required, that you assign a unique ISBN per format. A format is a Kindle eBook, an EPUB (used by B&N, Apple, Kobo, etc.), audiobook, hardcover, paperback. (See below for clarification.)
3. If you change the book beyond typos, you need a new ISBN.
4. There is no such thing as an eISBN. It is a myth.
5. You do not need an ISBN to publish via Amazon Kindle Direct Publishing (KDP) or Barnes & Noble Pubit (Nook). [Update: an ISBN is not required to sell eBooks via Apple iBooks, Kobo Books or Google Play Books if you use their self-service publishing system.]
What is an ISBN?
This stands for International Standard Book Number. You will see references to “ISBN-10” and “ISBN-13.” The 13 version is the same number but with a prefix to identify it as being for book publishing. You can show both numbers in the eBook or print book although only the ISBN-13 is necessary.
How much is it?
$125 for one, $295 for 10, $575 for 100 and $1000 for 1,000 numbers.
R.R. Bowker is the exclusive sales agent in the United States. They have a separate website devoted to this topic: www.myidentifiers.com.
Can you get a free ISBN?
Yes. Some publishing services firms will make it available for free (like Smashwords) while others may charge a nominal amount ($10 to $25 each).
What's the catch with a free ISBN?
Your name won't be listed as the publisher. ISBN's are registered to a “publisher” when sold so if you get one “free” or as part of a publishing package you buy from someone, the master record will show that company as the “publisher”.
If you want to be listed as the publisher you should buy your own ISBN.
When is it required?
It is up to the eBook distributor (like Smashwords), or the store selling your book or eBook, as to whether or not you need an ISBN. Example: Apple does not require an ISBN if you use their iTunes Connect to upload your eBook but Smashwords does require an ISBN if you use their distribution service to add your eBook to the Apple iBooks store.
What if you have published a book or eBook and you are changing it. Do you need a new one or can you use the old one?
Unless you are making minor changes like fixing typos, you need to assign a new ISBN. Adding more information or changing information requires a new ISBN.
Can you use the same ISBN for both Kindle and ePub?
As a general practice you can use the same ISBN as long as the content does not differ. In 2013 the Book Industry Study Group issued a clarification “embracing the concept of Proprietary Identifiers” such as Amazon's ASIN. The reality is that Amazon ignores the ISBN you assign to your Kindle eBook and instead assigns their own identifier called ASIN. This ASIN is what you and the public use to identify your Kindle eBook on Amazon when linking to the book. (See our article about how to create short links to your books.)
Do you put it in the eBook?
Yes, add the number to the copyright page of your eBook.
You can put all the numbers together on the copyright page or add the appropriate ISBN to the file type you are publishing. For example:
ISBN 13: 978-0-9999999-9-9 (Paperback edition)
ISBN 13: 978-0-9999999-9-9 (eBook edition)
If you buy…
Don't forget to return to your Bowker MyIdentifiers account and complete the information for each ISBN assignment. This is very important! The data is shared with book industry databases such as libraries and other retailers. Without it, they won't know the book exists should someone ask for it.
ISBN Video Training Course: ISBN (Numbers) Explained: Getting and Using ISBNs to Sell More Books
ISBN defined (Wikipedia)