Compendiums (or compendia) are a craftable resource that contain a sum of all modern knowledge of catkind. Just having them passively increases maximum science by 10 each. The formula is floor(compendia*10). They are used for endgame technologies past Navigation and for crafting blueprints.
As of 1.4.6.0 the compedium science cap will now be limited by 100% of building science, but this limit can be improved with Metaphysics, Cryptotheology upgrades, as well as Data Centers.
Compendia are used to research the following technologies:
Compendia are needed to research the Pyrolysis Workshop upgrade.
Crafting blueprints requires 25 compendia each.