[dredmorbius]

Books and academic publishing are related, if not identical, activities. I'd expect there'd be some correlation. And the data for academic publishing happened to be handy.

I suppose its probably possible to extract data from other sources -- ibiblio, perhaps -- for books, but that would involve work.

There's also the matter that the xkcd infographic shows long-term trends. It's entirely possible that there've been changes in recent years. Though that would also be pretty remarkable.

The huge increase in online access to content and information has a bigger effect though. I was purchasing a large number of books through the mid 2000s, far fewer since. The fact that numerous physical bookstores I could head to and shelf-browse have closed doesn't much help matters (I hate buying anything online, books included, especially via Amazon).