Identifiers (ISBNs, UUIDs, URLs)
Despite the introduction of ISBN-13 in 2007, it’s possible that the industry will run out of ISBNs if they’re used for the increasing number of ebooks being published. (Though we could keep adding book ISBN prefixes beyond 978–, starting with 977–.) For this reason and others, there is already discussion about new identifiers, including UUIDs (Universally Unique Identifiers, which are randomly generated strings of numbers and letters) and URLs (web addresses).
For now, the most common publisher question is whether to use a new ISBN for every format that an ebook is sold in.
Essentially, you need an identifier that works with your sales reporting system, whatever that may be. For most, that’s an ISBN. For now, we recommend, for each book:
One ISBN for every print edition (as you’ve always done)
One ISBN for the statically paged ebook (usually the PDF, but this ISBN could also be used for other static-paged formats, like page-based flash viewers)
One ISBN for the reflowable ebook (e.g. epub, but this could also be used for other reflowable formats like .mobi).
Reflowable ebook formats (such as epub, mobi, lit, pdb, etc.) are essentially interchangeable. They can be easily converted between each other. And in some cases, this conversion will be done on the fly at the distributor according to the user’s choice or device. And this is how it should be: a layperson should not have to know the file format they’re purchasing, it must just work. In the same way you probably don’t care which company’s petrol goes into your car, or whose flour the baker used in your everyday bread.
Moving forward, consider using UUIDs in addition to ISBNs, especially if you’re changing or upgrading your sales reporting systems. We won’t run out of those. (InDesign generates them automatically when you convert a document to epub.)
Also, consider using an International Standard Text Code to identify a single textual work an any of its versions (epub, PDF, paperback, hardback, large print, illustrated, pop-up, and so on).