International Standard Book Numbers (ISBNs) are unique 13-digit numbers that are used to identify and keep a track of your book internationally. If you are planning on selling and profiting from the sales of your book you should have an ISBN. Not only will the ISBN identify your book from another publisher’s title it will also be used to keep track of all the sales generated by your book. Each sellable format of your book should also have its own ISBN. So, you will need a different ISBN for the same title if it is available in different formats such as ePubs, paperbacks, hardbacks and audiobooks.
The ISBN must be inserted on the copyright page of your book. This is the page that comes after the title page. (You will find out more about this later.)
Not only is it important to have an ISBN for the National Library, it is also needed if you want your book to be on the Smashwords premium catalogue so that it will be distributed to e-retailers such as Kobo and Apple. Smashwords distributes the ePub format of your book to their e-retailers only if you have an ISBN.
Smashwords will give you an ISBN number. All you have to do is to assign the ISBN in Smashwords ISBN Manager from your dashboard. Note that you may use this ISBN only for the eBooks that you publish through Smashwords. For more information go to https://www.smashwords.com/dashboard/ISBNManager