Being new to the process of self-publishing, I am still trying to understand the nuances of ISBNs. Some sources say they are not required for e-books; some sources say they are "suggested" for e-books; some sources say that one ISBN can be used for both .MOBI and .EPUB versions of the same book; and other sources have indicated that each e-book edition should have a unique ISBN. Color me confused. Recently I was reading the free weekly newspaper that covers my town and I noticed a brief column by the director of the local library. (As an aside, does anyone know when "librarian" became "library director"?). With nothing to lose, I decided to send him an e-mail and ask if he could tell me anything about e-books and ISBNs. He responded very promptly, and what he had to say made me glad that I did assign ISBNs to the Kindle and Nook versions of the book I recently released. Rather than try to paraphrase his words, I'll just paste in his response: Thanks for reaching out. We definitely use ISBNs in our cataloging process, as do, I think, all other libraries. It's the best way to distinguish between different titles and different editions of the same title. However, we also catalog plenty of things that don't have ISBN's, like DVDs and CDs. We also offer e-books, but, as they are all accessed through third-party vendors, we don't' have anything to do with the cataloging of them. It is true that e-books don't necessarily need to have ISBNs in the same way print books do. However, Overdrive, the major supplier of e-books to (public) libraries, does require ISBNs for any e-book they sell. So that may be something to consider. I then asked, "How/where is an ISBN attached or displayed or associated with an e-book?" Frankly, I'm not actually sure of the process. But we would see the ISBN in the ebook store, and then it would get transferred to our catalog when we purchase it. It would be in the text of the ebook too. For me, the take-away is that libraries run on ISBNs. I think if I want to have any hope of getting an e-book version of something into library distribution, if the major supplier of e-books to libraries requires ISBNs -- I'll use ISBNs.