in Publishing
One ISBN per format, and why the rule exists
A hardback, a paperback, an ebook and an audiobook are four products to the book trade, and each needs its own ISBN for sound practical reasons.
The rule is simple to state. Each format of a book needs its own ISBN. A hardback has one, the paperback has another, the EPUB has a third and the audiobook a fourth. If there is a large-print edition, that has its own too.
Writers new to publishing sometimes read this as bureaucracy, or as a way of selling more numbers. It is neither. The rule follows directly from what an ISBN is for.
Why formats are separate products
An ISBN identifies something that can be ordered, priced and supplied. A hardback and a paperback of the same novel differ in price, in physical dimensions, in stock levels and often in publication date. A shop that orders one must receive that one and not the other. A library that catalogues one is describing a particular object on a particular shelf.
If both shared a number, every system downstream would have to guess which was meant. Orders would go astray, prices would display incorrectly and availability would be reported for the wrong thing. Separate numbers remove the guesswork.
Digital formats follow the same logic. An EPUB and a PDF are different files with different uses, and where both are sold as distinct products, each should carry its own ISBN. An audiobook is a different product again, with a narrator, a running time and a different set of retailers.
When a new ISBN is needed, and when it is not
Beyond format, the general principle is that a new ISBN is needed when a change would matter to someone buying or cataloguing the book. The obvious cases are a new edition with substantial changes to the text, a change of format or binding, and a change of publisher.
Some changes do not need a new number. Reprinting the same edition, even many times, uses the same ISBN. Correcting a handful of typographical errors does not make a new edition. Changing the price does not require a new number, and neither, generally, does a new cover on the same edition, though some publishers choose to treat a significant repackaging as a new edition for their own reasons.
The judgement in borderline cases is whether a buyer or a librarian would reasonably consider this a different product. If they would, it needs its own identifier. If the answer is unclear, the ISBN agency that issued the numbers can advise.
Ebooks and retailer identifiers
Some ebook retailers do not insist on an ISBN and will assign their own internal identifier instead. That identifier works within that retailer and nowhere else. It will not appear in the bibliographic databases that libraries and other retailers draw on, and it does not name you as the publisher of record.
For a book sold through a single store, that may be acceptable. For a book meant to be available widely, or to libraries, or recorded properly in national bibliographic data, an ISBN for the ebook is the more robust choice.
Keeping the family together
Separate numbers do not mean separate books in the reader's eyes. Most bibliographic systems and many retailers link formats of the same work so that a reader looking at the paperback can see that an ebook and an audiobook exist. That linking depends on the metadata being consistent: the same title, the same author name spelt the same way, and clear information about which format each ISBN represents.
It is worth keeping a simple record of every ISBN assigned, with the format, the edition and the date it was used. Numbers are issued in blocks and once one has been attached to a product it should never be reused for anything else, even if the original product was cancelled before publication.
Plan the formats before assigning the numbers, and the rest follows without much trouble.