The ISBN has a reputation as the thing that makes a book official. It is more boring than that: it is a catalogue number for a specific edition, so that a shop, a library and a distributor can order the same object. Whether you need one depends entirely on who you are asking to stock it.
The short version, by channel
| Channel | ISBN needed? | Notes |
|---|---|---|
| PDF sold on your site | No | You are the shop; you do not need a catalogue number to email a file |
| Lead magnet / free download | No | It is not being ordered |
| Amazon KDP, Kindle ebook | No | Optional; Amazon will identify it without one |
| Amazon KDP, paperback or hardback | Yes | KDP will assign a free one, with the catch below |
| IngramSpark, draft2digital print, most POD | Yes | They are distributing to shops that order by ISBN |
| Apple Books, Kobo, Google Play | Often optional | Check the store; many will issue or waive |
| Libraries, bookshops, most distributors | Yes | This is the original job of the number |
A book can therefore be fully published — sold, read, reviewed — with no ISBN at all, if the only channels are your site and Kindle. The number becomes mandatory the moment a third party has to order a physical copy they have never seen.
One number per format, not per book
The paperback, the hardback and the EPUB are three editions. Each gets its own ISBN. A revised edition that is not a reprint — new chapters, a new subtitle, anything that means a shopper might need to know which one they are holding — is another ISBN. A typo fix that does not change the product is usually not.
This is the rule people trip on when they buy one number and try to use it on KDP paperback and on the ebook. The systems will not all reject it, and the ones that accept it have now listed two different objects under one identifier. Downstream, that is how the wrong file gets ordered.
Free ISBNs, and the lock-in
KDP will give you a free ISBN for a paperback or hardback. Several other platforms do the same. Two things come with it.
- The imprint is theirs, not yours. On Amazon the publisher field becomes "Independently published" rather than a name you chose. For some authors that is fine. For anyone building a small press identity, it is the opposite of the point.
- The number cannot travel. A KDP-assigned ISBN identifies a KDP edition. You cannot print the same edition through IngramSpark under that number, and you cannot take it to another printer if you leave. Your own ISBN can go wherever you have the rights to send the book.
If you are publishing one paperback, only on Amazon, and you do not care what the imprint field says, the free number is a sensible way to spend nothing. If you intend to be in more than one shop, or to publish more than one book, buy your own.
Where you actually buy one
Each country has a designated agency. In the US it is Bowker; in the UK, Nielsen; other countries have their own, and the prices are not similar — a single ISBN can be a few pounds in one place and a three-figure dollar amount in another, with bundles that make the tenth book cheap. Buy from the agency for your country, not from a reseller adding a markup to Bowker's form.
You do not need a company. A person can be an imprint. The name you register is the name that will show as publisher, so pick one you are willing to see on the copyright page of book four.
What goes on the copyright page
The number itself, labelled, with the format if you have more than one edition in the same file's family. The year, your name, a rights line. A barcode on a print cover encodes the ISBN; that is a cover-design job, not a typesetting one, and print-on-demand platforms will usually generate it when you supply the number.
© 2026 Your Name. All rights reserved.
ISBN 978-1-NNNNNN-NN-N (paperback)
ISBN 978-1-NNNNNN-NN-N (ebook)That is sufficient. You do not need a Library of Congress number to publish, you do not need a CIP block, and you do not need to register the copyright in most countries for the copyright to exist — registration is a separate, optional, legal step that is worth looking up for *your* country rather than copying from a US blog post.
How this fits the rest of the pipeline
The ISBN is metadata. It does not change how you write or typeset the book. Put it on the copyright page of the manuscript, put it in the store form when you upload, and put it in the barcode of a print cover. The rest of the pipeline is structure, design, export and proof.
bukpress will typeset whatever you put on that copyright page; it will not issue you a number, and it should not. The number is yours, from an agency, or it is a platform's, from that platform. The file is just where it is printed.