score:19
"Affordable to the poor" is a slippery concept.
However, books became much cheaper in the first half of the nineteenth century. Notable causes of this included steam-powered printing presses, mechanical typesetting, pulp paper mills, and the railway distribution network. These factors allowed the publication of cheap paperback books, which seem to have been invented by the Tauchnitz family in Germany, and followed up by publishers such as Routledge in the UK, aiming at the railway traveller market.
Upvote:18
The Gutenberg-style press process initially output about 160 pages a day, which was revolutionary compared to about 12 you could expect out of a human copyist. This got polished to the point where by 1600 a press could output about 3,600 copies a day.
The next major advances came with the Industrial Revolution, starting with cast iron presses(in 1800) putting out about 480 pages an hour, then steam presses (in 1814) outputting more than a thousand pages an hour.
Each one of these advancements can be thought of as a similar % reduction in the price of printed works. With the new steam presses penny daily newspapers started to appear (vs. 6-cent).
However, we should backtrack a bit to the Bible and its importance to the history of printing. Most Protestant theology held that a person should read the Bible for themselves, rather than relying on a priesthood to interpret it for them. Inherent in this is the idea that a good Christian should be able to read the Bible, which means both literacy and owning a copy. This was more an ideal than a universal reality, but the Bible has always been the most-published book, and it was reality enough to actually impact the language in Protestant areas like Germany and England.