I would not get too excited; the titles mentioned in the posts above were published by Princeton University Press so I’d bet they are being regarded as textbooks (or at the very least academic titles).
The title mentioned by Engadget (here) is listed under the categories Cognitive Psychology and Study Skills. The ebook found by Zatz Not Funny (here) is listed in the categories political humor and political history & theory. I would not be surprised to learn that those titles were required for college classes somewhere, thus making them textbooks.
Amazon has been renting out Kindle textbooks since 2011, and they have long been expected to move the plan beyond textbooks to other content. In fact, this particular situation of someone finding a non-obvious textbook for rent has happened before.
Today's breaking news is basically a repeat of a story which cropped up in December 2011. Someone noticed that Amazon would rent the Kindle edition of a niche history book. I looked into it and I concluded that it was a textbook and thus not actually new story (Amazon had been renting Kindle textbooks for a couple months at that point).
Move along. There's nothing to see here.
image by Nemo