The basic answer is "with the invention and diffusion of the printing press across the 16th century" Jeremiah Dittmar has written some papers on the welfare impact of this change, and references some earlier literature with data on price and diffusion. (There's a working paper version of one paper here )The real price of books fell by about 90% over the 16th century, with the vast majority of the drop concentrated in the period before 1530. Uptake is harder to measure, and a slower process, but Dittmar cites Peter Clark (1979) who finds that the share of households owning books, from probate record in Kent, rises from about 15% in 1550, to about 50% in 1640. The same kind of pattern holds for most of Northern Europe (Germany, the Netherlands, France, etc...) and is slower the further away you get.