The Library! So big it doesn't need a name, just a great big "The!"
— The Doctor, "Silence in the Library"
Lucifer walked slowly along the surface of the library's dark aisle of ethereal water that was connected to cosmic engines located at Dragylon's core. His jewel-encrusted wings fluttered behind him like the robes of a scholar while he glanced at the towering bookcases to the left and right of him. He strode through shafts of light descending down from the sphere's atmosphere through certain panels that were open on each side of the library's vaulted roof, eyes flashing brightly each time he passed through the pockets of thick shadows, the darkness consuming all but the muscled outline of his golden form like a solar eclipse. The library consisted of rolled scrolls of ivory parchment stuffed into open sleeves, with each sleeve stacked on top of one another on the shelves. Small flaming symbols of angelic and cherubic origin hovered in the space of each sleeve's open circle, serving as a coded filing system created by his scribe Medius. Lucifer's own voice whispered at him from behind the scrolls' fiery symbols as he walked past them, swirling around him in cosmic drafts of devilish diatribes and prideful proclamations.
Among the beauties and wonders of the city, the Library of Arcane Knowledge stands out for its purity of form and its purpose. A great crystalline block with no apparent entrances, this place supposedly serves as the repository of all human knowledge - past or future. All magickal workings, all inventions, all philosophy, science, poetry, literature - in short, everything humans ever thought of or ever will think of can be found inside the library. Or so they say.
—Mage: The Ascension - The Book Of Worlds
Deep inside the Impossible Fortress, according to some profane accounts, lies Tzeentchs fabled Hidden Library. This infinite collection of tomes, scrolls, and parchments of every kind contains every scrap of knowledge and thought ever recorded; stories written and unwritten; histories true and alternate; and accounts of futures potential, actual, and imagined. Many of the volumes are so weighty with knowledge that they gain a sentience of a kind and spend centuries chattering to passersby, arguing with one another, rewriting themselves, and then reorganising their placement accordingly. Magical chains of warpflame help to protect the books and bind them in place. Horrors serve as grotesque librarians and work tirelessly to re-shelve the works, catalogue the collection, and maintain what passes for order in the Impossible Fortress, though as the concept itself is anathema to the Great Mutator, no mortal could possibly fathom such a design.
—Black Crusade: The Tome Of Fate