Note that this doesn't tell us how many are being sold, but it does show that Spanish publishers are seriously investing in ebooks. Well, they're investing in PDF (70% of the ebook ISBNs), which I'm not sure qualifies as an ebook. Epub only made up a fifth of the ebook ISBNs, with Kindle counting for another 5%.
Another source did some math on the stats and they reported that ebooks made up 22% of ISBNs registered last quarter and 17% for the 2011 calender year.
I have looked for similar numbers for the US market, and I've been told that it's difficult to estimate. I was also told that the US numbers would only be approximate, so I would be cautious about reading too much into the Spanish numbers.
If I get the US numbers (or anywhere else), I will amend this post. On a related note, does anyone know what type of content is going into the PDFs mentioned above? Based on the general spread of all the ISBNs registered, I'm guessing technical and educational, but I don't know for sure. But whatever it is, clearly the content isn't being released as Epub.
What really interesting about these numbers is that they don't jive with the Bowker survey results. Amazon accounted for 30% of purchases and yet publishers are only registering 5% of their content in that format. I think they could be missing something (besides the probability that Spanish ebook buyers are getting content from over the border).