According to BISG:
- If you release your e-book in both Amazon and EPUB formats, you need two separate ISBNs.
- If your EPUB is available DRM-free, with Adobe DRM, with B&N DRM, and with Apple DRM, those can share the same ISBN.
- Except that if the Adobe DRM allows limited printing in one case but not in another, those require separate ISBNs.
- In contrast, if the Adobe DRM gives a time limit (library loan, ARC, etc.) that can use the same ISBN as an unlimited version.
- Implied: if you correct a typo and re-release, you need all-new ISBNs.
The BISG press release quotes Hachette's Phil Madans, who chaired the group: "some even reported thinking ISBN assignment should be optional--a 'nice to have'. This, clearly, would not work." Um, clearly it is working.
Pardon my cynicism, Big Publishing, when you come up with a recommendation that is of trivial consequence for publishers who buy ISBNs in blocks of 10,000 but is a significant financial burden to the self-publishing author. I'm reminded of Anatole France's observation, "The law, in its majestic equality, forbids the rich as well as the poor to sleep under bridges..."