[layer8]

It’s not uncommon for an ISBN to have been assigned multiple times to different books [0]. Thus “all books in ISBN space” may be an overstatement.

There’s also the problem of books with invalid ISBNs, i.e. where the check digit doesn’t match the rest of the ISBN, but where correcting the check digit would match a different book. These books would be outside of the ISBN space assumed by the blog post.

[0] https://scis.edublogs.org/2017/09/28/the-dreaded-case-of-dup...

[mormegil]

And possibly not even assigned at all. I looked at the lowest known ISBNs for Czech publishers and a different color stood out: no, https://books.google.cz/books?vid=ISBN9788000000015&redir_es... is not a correct ISBN, I'd say :-) (But I don't know if the book includes such obviously-fake ISBN, or the error is just in Google Books data.)

[Finnucane]

Publishers buy blocks of isbns based on expected need, how the actually assign them may be arbitrary.