This is an important matter that has created quite a bit of controversy in the industry. From the press release:
The Book Industry Study Group (BISG) announced today the publication of a new Policy Statement detailing best practices for assigning ISBNs to digital products. Developed over the past 18 months within BISG’s Identification Committee, BISG Policy Statement POL-1101 addresses the critical need to reduce product identification confusion in the market place in order to provide the best possible consumer-level purchasing experience.
BISG encourages all member companies and other industry stakeholders to download the Policy Statement online at http://www.bisg.org/what-we-do-cat-4-policy-statements.php and work toward adopting the suggested guidelines as soon as practical, with a target for new product introductions of no later than March 2012. The best practices are applicable to content intended for distribution to the general public in North America, but could be applied elsewhere as well.
The Policy Statement has been endorsed by BookNet Canada, a not-for-profit agency dedicated to innovation in the Canadian book supply chain, the National Information Standards Organization (NISO), where content publishers, libraries, and software developers turn for information industry standards that allow them to work together, and IBPA, the Independent Book Publishers Association.
The following excerpt starts on page 6 of the 12-page Policy Statement:
“Separate ISBNs should be assigned to all unique Digital Books for ordering, listing, delivery and sales tracking purposes. In general, there are three major factors that determine the need to assign unique ISBNs to Digital Books.
If two digital books are created, one an exact textual reproduction of a Physical Book and the other an enhanced version that includes video, audio, etc., then the two Digital Books are unique and different products, and each requires a unique ISBN.
If an EPUB format, a PDF format and a Mobi format (among others) are created, each format should be assigned a unique ISBN. This is similar to creating a hardcover and paperback edition of a Physical Book and should follow the same rules regarding ISBN assignment.
(Note: When the application of DRM software is part of the transaction with the Consumer (as frequently happens in the US) it does not constitute the creation of a new format as the term is being used in this Policy Statement. In this case, DRM is not a format: it is a wrapper around a product. An EPUB file with DRM software applied is still an EPUB file, a PDF file with DRM applied is still a PDF file. In this case, DRM is not part of the product, it is part of the transaction. An ISBN is a product identifier, not a transaction identifier.)
— Usage Rights
If a Digital Book is made available with different usage rights in different markets (e.g. adjusting the usage settings so that printing is allowed in the version going to the education market, but not in the version going to the retail market), each version should be assigned a unique ISBN.
(Note: As described in the note under ‘Format’ above, usage rights specifically applied to a publisher’s Digital Book using DRM software, such as Adobe Digital Editions or Apple FairPlay, as part of the transaction between the vendor and the Consumer does not fall into the above category and does not require the assignment of a unique ISBN.)”