XX Chapter of Adeptus Astartes does not maintain a Librarium. However, they do employ the weapons of the warp, esoteric and arcana, forbidden lore and far seeing arts.
This is the work of a legion Warrior Mystic, psychic masterminds and warrior gods as individualistic and creative as chaos. Each mystic has an apprentice if a worthy candidate is found, and millennia of lore is passed in secret, verbally, in remote planets and webs of the galaxy.
Mystic Alpharius, master of the farseer, brother of the moon, student of the old one and leader of Stealth team Argolith.