FWIW
The Atlantic covered this story much better (http://www.theatlantic.com/technology/archive/2013/07/the-ho...) and their coverage included two additional charts. One corresponds to your criticism of adjusting for titles vs versions. In that chart, the drop off in the mid 20th century is still evident but not as extreme. In the second chart, they account for differences in the number of books published per decade using third party sources. In that version, the divergance is actually much worse.
I don't think there were any responses to your criticism of the categories.