A unique 13-digit number assigned to each edition of a book, like a Social Security number for your publication.
ISBN stands for International Standard Book Number. It's a 13-digit identifier assigned to every edition and format of a book, so retailers, libraries, and distributors can track and order it accurately. A paperback edition gets one ISBN, the ebook gets a different one, and the audiobook gets yet another. Without an ISBN, your book essentially doesn't exist in the traditional supply chain.
If you're publishing a book and want it available in bookstores, libraries, or any major retailer, you need an ISBN. It's the barcode that makes your book findable. Self-published authors sometimes skip it for ebook-only releases, but having one signals professionalism and opens distribution channels that are otherwise locked.
ISBNs were introduced in 1970 and originally had 10 digits. They switched to 13 digits in 2007 to accommodate the growing number of publications worldwide.
The first UK edition, first US edition, paperback, hardcover, and ebook each have different ISBNs. The original UK hardcover ISBN is now associated with a book worth thousands of dollars to collectors.
Each format (hardcover, paperback, ebook) and each significant new edition needs its own ISBN. Budget accordingly.
Free ISBNs from platforms list the platform as the publisher of record. If you want your own imprint name, purchase ISBNs directly from your country's issuing agency (Bowker in the US).
A single ISBN costs $125 in the US, but a block of 10 costs $295. If you plan to publish more than one book or format, buy in bulk.
Look up the ISBN for a book you own and search it on a site like isbnsearch.org. Find every edition and format listed under that title. Notice how each has its own ISBN. If you're planning to publish, calculate how many ISBNs you'd need for all the formats you want to release.