Heads up: This is a plain-language summary, not official policy, and rules change often. Always confirm the current requirements with the official source linked in each section. BookZeta does not publish on your behalf and can't guarantee any marketplace will accept your book.

An ISBN (International Standard Book Number) is the unique identifier for a specific edition and format of a book. The key word is format: each format — eBook, paperback, hardcover, audiobook — generally needs its own ISBN, and a major revision can warrant a new one too. The ISBN is how stores, libraries and distributors tell your editions apart.

Do you actually need to buy one?

Not always. There are two common routes, and which one is right depends on how you plan to publish.

• Use a free, store-assigned ISBN. Many marketplaces (including KDP) can assign a free ISBN for use on their platform. This is the simplest option and costs nothing. The trade-off is that the store is typically listed as the registered publisher of record, and that free identifier is generally tied to that platform.
• Buy your own ISBN. Purchasing your own lets you be listed as the publisher, and you can use the same identifier consistently across multiple stores. This is the more "professional publisher" route and is often preferred if you're distributing widely or building a brand. ISBNs are frequently sold in blocks, which works out cheaper per number if you'll publish several titles or formats.

There's no universally "correct" choice — a single eBook on one store may be perfectly fine with a free ISBN, while a print title going to bookstores and libraries usually benefits from one you own.

Where to get one

• United States: Bowker / MyIdentifiers is the official ISBN source for U.S. publishers.
• Other countries: use your national ISBN agency. Every country has one, and many issue ISBNs free or at low cost. Search "ISBN agency" plus your country to find the right one — buying from your own country's agency is important, because ISBNs are issued regionally.

A few details that trip people up

• One ISBN cannot cover multiple formats. If you sell an eBook and a paperback, that's two ISBNs.
• An ISBN identifies the book, not the cover. A new cover with the same content usually doesn't need a new number; a substantially revised edition might.
• Audiobooks may use an ISBN or a platform identifier such as an ASIN/ACX identifier, depending on where you distribute — so check the requirements of your chosen audio platform.

If you're just testing the waters with a single title on one store, a free assigned ISBN keeps things simple. If you expect to grow a catalogue or distribute everywhere, owning your numbers gives you more control. Either way, confirm the current options and pricing with the official source for your country before you buy.