I don't have data as detailed as the Spanish ISBN agency released, but I do have a figure for the US market.
There were a total of 1,555,790 ISBNs assigned to book products (ebooks, audiobooks, print books, and some video) in 2011 (that Bowker knows of – not all publishers report back to Bowker what they assign).
Of those, 183,183 were assigned to ebooks. Which is 11.77%.
The curious detail about this is that in 2011 the Spanish market saw around 17% of ISBNs be assigned to ebooks. So that leads to the obvious conclusion that the Spanish ebook market is growing faster, right?
No, and that's where this gets interesting. I was told, long before I got the 12% figure, that the number would be worthless. ISBN registration in the US book market is distorted by a couple of factors so the registrations don't reflect the real number of titles being released.
The main reason for the 12% figure was that some POD publishers have been basically (as I see it) spam registering ISBNs for all the POD editions of all the public domain books they print. These aren't necessarily new books, and TBH it would be a stretch to describe them as being actually in print. But the ISBNs are being registered.
So while I can say that the Spanish ebook market is growing as a percentage of the total Spanish book market, I really don't know how it compares to the US market.
Thanks to LJN Dawson for asking Bowker!