[gmcharlt]

ISBNs are messy.

The International ISBN Agency coordinates assigning ISBN ranges to national agencies, who in turn will assign subranges to publishers. The publishers in turn assign specific numbers to their own works. However, the international agency does not itself maintain a universal database of assigned ISBNs - the most it operates is a global database of publishers and their assigned ranges. And since it's the publishers who are assigning numbers from their allocations, various errors can crop up, including reusing ISBNs for different works and failing to issue distinct ISBNs for different formats. (For example, if you publish hardcover, paperbook, and ebook versions of a book, you should assign three ISBNs. That rule is not always observed.)

Also, libraries hold many books that long predate ISBNs; it wasn't until 1965 that the immediate predecessor of the ISBN, the SBN, was a twinkle in a bookseller's eye.

[bambax]

Yes.

And while in most countries you can't properly publish a book without an ISBN (ie, have it sold in bookshops), you can publish a Kindle book without it (if you opt to only offer the ebook).

That leaves a huge part of publications completely out of the system. Kindle-only books are on Amazon servers and nowhere else.

[wayathr0w]

>while in most countries you can't properly publish a book without an ISBN (ie, have it sold in bookshops)

I'm quite skeptical of this, given the amount of books I've personally seen published in recent decades without ISBNs, along with the limited & haphazard attempts to regulate what it means to 'publish' something or even to be a 'proper' bookseller. But if you have some experience I don't with this, I'm interested in hearing about it.