Prior to the invention of the printing press, writing (and copying) books was hugely more expensive than reading. It was far easier to standardise on language, and train readers in that common language, than to produce books in each localised language.
In labour time, a book represented 1/3 - 1/2 a man-year of labour, for the scribe alone. Plus 300 calves for the vellum. About $0.5 - $1 million.