The way it works is simple, there are 2 primary ways to get symbols.
First is to buy them from special villagers for anywhere from 1 to 64 emeralds.The villagers look like the magenta robe wearing priests but have a gold strip down their front.
Second is to find lost library dungeons that only spawn in mystraft worlds and are a bit rare. These spawn on the surface and look like stone brick boxs filled with book cases, not hard to miss.
So you can either hunt down a village with the right villager and farm emeralds or go diving in randomly created mystraft worlds that almost always have some instability to varying degrees. As you can still make random worlds with descriptive books, you just cannot get the symbols out of them.