Check out this helpful post from the How to eBook blog on the topic of book marketing QA around ISBN numbers and self-publishing
Question: I am pondering publishing a book through Ingram Spark and would like the book in bookstores. When should you use ISBN numbers to self-publish?
Answer: Every book you publish, whether self-published or done with a major publisher, should have a separate ISBN number. In fact, each version of the book you publisher should have a separate ISBN number: hardcover, paperback, audio, ebook.