If you want to publish a book under your own imprint, you’ll need to buy an ISBN for each format you produce. That is, you’ll need one ISBN for the paperback, another for the hardcover, another for the Kindle version, and another for audiobook.
Bowker is the only authorized seller if ISBNs in the US. ISBNs can be expensive. As of February, 2022, Bowker charges $125 for a single ISBN, $295 for a pack of 10, and $575 for a pack of 100.
You can avoid buying ISBNs altogether if you publish through KDP and you don’t mind your book’s official publisher being listed as KDP. If you publish through most other sources (such as IngramSpark), and/or you want your book’s publisher to be listed under the name of your custom publishing company, you’ll need to by an ISBN for each format you intend to release.
Also note that substantially revised editions of the same work need separate ISBNs. For example, let’s say you publish a paperback called How to Repair Computers. Five years later you produce a second, updated edition covering new technologies. The first and second editions of the paperback must each have their own ISBNs.
You do not need a new ISBN if you’ve only made minor corrections to an existing work.
Bowker Add-ons: Barcodes and Copyrights
When you go Bowker’s site, you’ll notice options to buy barcodes and copyright registration services. You don’t need to spend money on either of these. The cover template generators from KDP and IngramSpark (and likely many others) have an option to include a barcode on your back cover.
In the US and most other countries, you do not need to register your copyright. Your work is implicitly copyrighted when you put your name and a copyright notice on it. The notice generally looks like this:
© 2022 Your Name.
You may want to consider copyright registration if you’re worried about your book being pirated. This is a real problem with high-priced technical books, but generally not with novels, memoirs, or other titles that sell for less than $35 and don’t contain valuable technical knowledge.