in Publishing
What an ISBN is, and what it isn't
An ISBN is a product code for one version of one book. It is not copyright, not a licence to publish and not a guarantee that anyone will stock it.
An ISBN, the International Standard Book Number, is a thirteen-digit identifier. It exists so that a bookseller, a wholesaler, a library and a publisher can all be sure they are talking about the same thing. That thing is not the text of a book. It is one specific edition of it, in one specific format, from one specific publisher.
That distinction matters more than it first appears, and most of the confusion around ISBNs comes from treating the number as something grander than it is.
What the number does
The book trade runs on data. When a shop orders a title, the order travels through systems that match it by ISBN. When a library catalogues a book, the ISBN ties its record to the right physical object. When a retailer displays a price, a cover and a publication date, that information has usually been attached to an ISBN somewhere upstream and passed along.
The number itself carries a little structure. It begins with a prefix, then identifies a registration group (broadly, a country or language area), then the publisher, then the individual title, and ends with a check digit that lets software catch typing errors. Since 2007 all ISBNs have been thirteen digits long, which is why the barcode on the back of a book and the ISBN printed above it now match.
The part that identifies the publisher is significant. An ISBN records who published this edition. Whoever holds the block of numbers it came from is, as far as the trade's databases are concerned, the publisher of record.
What it is not
An ISBN is not copyright. In the UK, copyright in a literary work arises automatically when it is written down, without registration or any number at all. Assigning an ISBN adds nothing to that protection and taking one away removes nothing.
It is not a legal requirement for publishing. You can print and sell a book in the UK without one. What you lose is easy access to the systems that most retailers, wholesalers and libraries use, and in practice that is a large loss for any book meant to be sold beyond a single venue.
It is not proof of quality or of anything having been checked. The agencies that issue ISBNs do not read the book. They issue numbers.
It is not a listing. Having an ISBN does not put a book in any shop or on any website. It makes the book identifiable when someone decides to stock it, which is a separate matter.
And it is not the book in the abstract. A novel that exists as a hardback, a paperback and an ebook is, to the trade, three products. Each needs its own number, because each has its own price, its own availability and sometimes its own publication date.
Why the details are worth getting right
Because so much else is keyed to it, an ISBN is hard to change once a book is out. Metadata, reviews, sales histories and library records all attach to the number. Assign the wrong one, or reuse a number that belonged to something else, and those records collide in ways that take time to untangle.
It is also worth knowing who is named as publisher. If an ISBN is supplied by a third party, that party may appear as the publisher of record, which can matter later for distribution, rights and simply for how the book presents itself.
None of this makes the ISBN complicated. It is a product code with a check digit. Treated as exactly that, it does its job well.