This particular category was an especially tough call, because there have been so many great innovations even in this past century. In 1452, Guttenberg's movable type printing press , the spiritual and technological precursor to the Internet, increased the spread and flow of information by orders of magnitude, and in so doing helped catapult the West out of the Dark Ages and into the Renaissance. Literacy rates went up, the sciences flourished, and the whole world changed as information and disinformation, propaganda and philosophy, ideas and discourses were made available in an unprecedented way. The development of mass production paper-making techniques. Paper was brought from China to Italy in the 12th C. but was thought too flimsy for books.
Vellum is extremely durable.
For books that took more than a year to produce, paper was too flimsy. However, for print books, vellum was too costly to produce. The development of oil-based inks. These had been around since the 10th century, but smeared on the vellum used to make books. The religious manuscripts used an egg-based tempura.