It's also not clear if those numbers are just 'trade' books (i.e., books sold through regular bookstores), or includes academic titles, textbooks, reference boo

It's also not clear if those numbers are just 'trade' books (i.e., books sold through regular bookstores), or includes academic titles, textbooks, reference books, etc. I mean, there's a lot of stuff published that has a fairly specialized or otherwise limited market. Probably the overwhelming majority of books fall into categories that are never going to sell 100K or more no matter what.