[yuribro]

I can only assume it's implied that the type of books this is about are philosophical (in the most broad sense, including self improvement) books, and so it makes sense to talk about religious books as examples?

People read for a variety of reasons, from professional books to books purely for entertainment. It also has to be said that while in those "pre-modern" times books were the most common medium, today there are many different ways to consume knowledge: newspapers, journals, blog posts and websites, graphic novels as written material, and we also have radio, tv, film, podcasts and many other types of source which serve the same role as books once did.