Found this interesting paper/report. About 120-150 books were written in the whole Europe in the 7th century, about 20 millions books printed in the 18th century. A printing press invented by Johannes Gutenberg in 1440 changed everything - a sort of Bitcoin of that time. A great price deflation followed, the second plot: book prices dropped by massive 85-90% during the 15th century.

In the report one can also find more interesting facts, e.g, how the literacy changed over time.