Fascinating. It allows for some interesting observations when you as I zoom in on this one (sadly no direct links to coords/zoom level) https://archive.anarchy.cool/maps/isbn.html You can find publishers like Hueber Verlag[1] in the eastern part of the German language section. They spread their ISBN numbers in a pattern with something like 1360000 between them (I know, ISBN having a checksum leads to gaps in the numbering), which generates a repetitive pattern with plenty of empty space. It is so wasteful on this huge chunk they have.

Are there no rules on how publishers have to assign their numbers? Just so they could hand back an unused block if they don't need it any longer.

[1] I can see how publishing learning material in 30 languages can give people "ideas" when assigning ISBN numbers https://de.wikipedia.org/wiki/Hueber_Verlag