What an ISBN actually does
An ISBN identifies a specific edition of a specific book in the global supply chain — it is how a bookshop, a library, a distributor and a retailer all agree they are talking about the same object. Print and ebook are separate editions and need separate identifiers. So does a hardcover, a paperback, a large-print edition, and a second edition with new content. One book routinely needs three or four. This is the first thing that surprises people: an ISBN is not a book, it is a format of a book.
The free platform identifier
A print-on-demand platform will assign one at no cost, and the book gets a valid identifier and can be sold. Nothing about it is fake or second-class. The catch is who is recorded as the publisher: the platform is, not you. The identifier is also tied to that platform, which means you cannot take it with you to another printer or distributor — moving the book means a new identifier and, in practice, starting the edition’s record over. For one book on one platform, none of that may ever bite. For an author building something, it eventually does.
Your own ISBN
Bought from your territory’s registrar, an owned ISBN records your name or your imprint as the publisher of record. It travels: the same identifier works with any printer, any distributor, any territory, and it stays yours if you change every other decision about the book. Buying in blocks lowers the per-unit cost considerably, which matters as soon as you intend to publish more than once — and given that one book needs three or four identifiers, the second title arrives sooner than most people plan for.
When the free one is the right answer
A single book, sold through one platform, with no bookstore or library ambition and no plan for a second title. In that case an owned identifier buys you optionality you will not use, and the money is better spent on editing — which is the single highest-return line item in the entire production. There is no virtue in owning an identifier for a book whose real constraint is that chapter four does not work.
When it is worth owning
You intend to publish again. You want bookstores or libraries to be able to order it, which often means the publisher of record cannot be a competing retail platform. You are building an imprint, or the book carries a business or an institution behind it whose name should appear as publisher. You want the freedom to change printers without re-registering the book. Any one of these makes ownership worth it; two or more make it obvious.
The part people miss: metadata is not one thing
Categories, keywords, description and comparable titles determine whether anyone finds the book at all — and they are not identical across retailers. Category taxonomies differ, keyword fields differ in number and length, and some retailers weight the subtitle heavily while others barely read it. Filling one form and copying it everywhere leaves most of the available discoverability unused. An owned ISBN attached to lazy metadata is a well-titled deed to an invisible house.