title_body: Under what conditions should I apply for ISBN numbers for digital works?
upvoted_answer: Note the following -
  
  
  A print book ISBN cannot be reused for the eBook
  Separate ISBN for each type of format. Kindle, ePub, audio, print (paperback, hard cover), etc. are separate formats and each requires its own ISBN.
  If we change the book beyond typos and we need a new ISBN.
  We do not need an ISBN (its not mandatory) to publish using Amazon Kindle Direct Publishing (KDP), Apple or Kobo. Each stores assigns its own code
  Unless we are making minor changes like fixing typos, we need to assign a new ISBN. Adding more information or changing information requires a new ISBN.
  Make sure that complete information for each ISBN assignment is given to your ISBN Authority. 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.
  Author can put all the numbers together on the copyright page or add the appropriate ISBN to the file type we are publishing. 
  For example:
  ISBN 13: 978-0-9999999-9-9 (Paperback edition)
  ISBN 13: 978-0-9999999-9-9 (Kindle edition)
  ISBN 13: 978-0-9999999-9-9 (ePub edition)
  
  
  Source: http://www.sellbox.com/isbn-essentials-an-faq-for-ebook-publishers/ &amp; Internet