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. They are used for endgame technologies past Navigation and for crafting blueprints.
Compendia are used to research the following technologies:
Compendia are needed to research the Pyrolysis Workshop upgrade.
Crafting blueprints requires 25 compendia each.