Before the printing press, all books had to be hand written. Even a small book could take months to hand write, and a large book like the Bible could take years. In Europe most books were made by the Roman Catholic Church with monks. Large monasteries had rooms called scriptoriums where up to 50 scribes would write as a single reader dictated a text. With each book being individually written, mistakes and differences between versions were common.
Since every book had to be written by hand over months or even years, books were rare and extremely expensive. In terms of today's dollars, a single book would cost tens of thousands of dollars! As you can imagine due to the rareness and high cost of books, few people could read. With books being so difficult to produce and largely done by the Catholic Church, political and religious authorities had great control over what books were made.
Consider the two contrasting images on the right of how books were made before and after the printing press.