I say this as someone who grew up in Spanish libraries and book shops, surrounded and immersed in Spanish books, so it feels a bit strange to see the tiny bit we occupy in the world map here.
Example: Germany publishes five times more books than the Netherlands [2], and Spain publishes twice as many books as the Netherlands. However, in visualizations, Germany appears similar to the Netherlands, while Spain and Mexico do not aligned with the high-level labels [3].
[1] https://annas-archive.li/datasets
[2] https://internationalpublishers.org/wp-content/uploads/2023/...
[3] https://software.annas-archive.li/AnnaArchivist/annas-archiv...
I had the exact same question, and I do have a completely unsupported theory. There's one large block that appears to be Argentina, or possibly Peru, although their titles are on the fringes of the large block. The block is otherwise unlabled, no name sitting at the center of the block like you see with the other major ones. I would be slightly surprised if it were entirely argentina, but it would make a lot of sense if that block were Spanish.