LOOK inside any book published since 1970 and you will find a number. But perhaps not for much longer. The International Standard Book Number (ISBN), invented in Britain in 1965, took off rapidly as an international system for classifying books, with 150 agencies (one per country, with two for bilingual Canada) now issuing the codes. Set up by retailers to ease their distribution and sales, it increasingly hampers new, small and individual publishers. Yet digital publishing is weakening its monopoly.

Publishers who were in at the beginning got great blocks of ISBNs. Many have plenty still in stock. Some countries, including Canada, Hungary and Croatia, make them free to bolster book publishing. But in Britain, America and Japan, where ISBNs are needed for any hope of mainstream publication, they are costly.

In Britain Nielsen, a media-data giant that is also the country’s sole ISBN agency, issues sets of ten for £126 ($190). Americans can pay $125 for a one-off number to R.R. Bowker, another data provider, but subsequent editions require another fee. When Valerie Merians and her husband started Melville House Books from their kitchen in New York 12 years ago, having to buy the codes was “intimidating”, she says.

But publishing is changing. Self-published writers are booming; sales of their books increased by a third in America in 2011. Digital self-publishing was up by 129%. This ends the distinction between publisher, distributor and bookshop, making ISBNs less necessary.

Alternatives are appearing, too. Amazon has introduced the Amazon Standard Identification Number (ASIN). Digital Object Identifiers (DOI) tag articles in academic journals. Walmart, an American supermarket chain, has a Universal Product Code (UPC) for everything it stocks—including books. Humans are also getting labels: the Open Researcher and Contributor ID system (ORCID) identifies academics by codes, not their names. And ISBNs are not mandatory at Google Books.

ISBN agencies argue that their system ended confusion: until the era of the universal number dawned, identifying a book was hard. That may be true in the analogue world, but in the digital realm what matters is not the number that a publisher gives a book, but how easily it can be downloaded and for how much.