Before the invention of a printing press, books were copied mainly in monasteries or in commercial scriptoria. In either place, scribes wrote them out by hand. (Apprentice Wizards would have to copy each book they have, taking a year or two of their lives just doing that). These scribes often applied beautiful illuminations to their text. Books were therefore a scarce resource. (Books are just bound scrolls, so the same thing would apply to scrolls.
A printing press expands this. No matter who developed it (Chinese, Korean, or German) it came from a press (used to squish fruit, grapes, olives). Originally pages were cut from single blocks of wood. Each block was good for approximately 10-25 pages, making printing presses faster than a manual process but still fairly slow. The invention of movable type (first of clay, then of metal) speeds this process along. The Chinese/ Koreans invented this process first, but it was of less usefulness given the word/symbol nature of their languages. In Germany the process was re-created independently (though there are arguments about it not being independent). Guttenberg, being a goldsmith before a printer, was able to carefully craft movable type. Given the alphabetic nature of European languages, movable type was vastly more useful.
Once movable type was created, it multiplied the speed of page reproduction by a factor of 50. With advances in paper placement (which was originally done by hand, but later had mechanical assistance), this number increased.
On Earth, book content has been centered on religous texts, either main foci of religious thought, commentary, or support (such as a hymnal). Following a close second is the works required by government or nobility to rule. These two powerful groups Religions and Government have a vested interest in controlling the words printed. By controlling books, they can control ideas, or at least limit their easy disemination.
The supplantation of hand copied manuscripts with printed works was not received with unanimous encomium. The papal court contemplated making printing presses an industry requiring a licence from the Catholic Church, making sure they published works only authorized by the Church. Similar resistance was later encountered in much of the Islamic world, where calligraphic traditions were extremely important, and also in the Far East.
Despite resistance, the printing press spread across the European world quickly once it became known. For the first time, anyone could read the Bible and no longer required clergy to interpret the word for them. The press also allowed the distribution of Bibles in the venacular (local language), further tearing down the clergical control over religion. The press was also key in Protestant Reformation.
The Press also led to the establishment of a community of scientists (previously scientists were mostly isolated) that could easily communicate their discoveries, bringing on the scientific revolution.
Though late in catching up, occultists also published their works. Admittedly it was two centuries after The Church and Science had been doing so, but Occultists have always been an insular and secretive bunch (especially since being an occultist had a death penalty in many places).
In Korea and China, there were no texts similar to the Bible which could guarantee a printer return on the high capital investment of a printing press, and so the primary form of printing was wood block printing which was more suited for short runs of texts for which the return was uncertain.
Some credit the printing press with giving Europe the technological and communication edge over Eastern countries in the end, one of the major questions in world history.
So what would happen if someone created a printing press in your game world?
Kerren has always had the printing press. These presses are manually opperated but have technological advantages that begining manual presses never would have.
In Arth, this is happening right now, as a current Meta-Plot. On Arth, Magic users are the primary producers and consumers of books. One of their own, utilizing the theory that matter was nothing more than magical energy in a different state, applied magical theory to material objects. Ariones MageSmith of Antioch, now known as Ariones Impressor created a number of items before the Guild took action against him. (If his theories were true, eventually there would be no need for wizards or magicians). He developed the theory of optics, improved metallurgy, and created a printing press. The Mage's Guild originally held seige around his workshop, awaiting approval of the Guild Council to take action. In those intervening eight days, Ariones printed 50 manuals of spells of the 8th rank (basic apprentice spells). It would of taken one scribe 64 days to do one basic book. He presented them as a gift to the Guild and presented them with several copies of his Magic Theories: A magical material world. The Guild realizing the advantages of having their works mass produced (more books, more magic users). They also realized the potential revenue to their guild. The accepted Ariones's work as Guild Acceptable. Eventually a new subguild, the Impressors (as in Printing Impressions) is created to take care of bookmaking and developing new items based on a magical material world. On my world, a technical revolution of sorts has begun. It has only been going for about a year at this point.
Of course, magic is a skill based system on Arth. So your religions might be more upset by the possible existance of 'outside ideas' existing; or governments might try to control the
easy access to information'. The existance of cheap books allows information on a number of subjects to be quickly dispersed. Someone could go from ignorant to the local expert on a subject after a day or so of reading.. The printed word will rock your world.
So what would happen on your world?