It is shown to be underground full of enchanted books and has catacombs and tunnels so long and winding that even Marian, Keeper of it for years, hasn't seen them all.
It contains numerous books through the years about the Caster world such as cast books (including books from Babylonian times and from Assyria), Dark and Light talismans, caster scrolls, binding books, objects of power, and etc. The cast books and caster scrolls can't be touched by mortals or it will burn their hands. Olivia Durand, was originally being trained to be the next fellow Keeper of the Caster library (as Marian was the current one being in charge of it) until she was expelled from her soon-to-have-been position for participating in a Cast with Ethan, since Keepers are not to become involved in such a thing, only learn about it. There are only a few known cast books and scrolls such as Casting, A Brief Historie; Castying to Creyate & Confounde, Casting the Confederacy, The Complete Guide to Poisonous Herbiage and Verbiage, Toes: To Caste Hair on Your Maiden's, Tongues for Binding and Casting, Toffee: Casts Hidden Inside, and Destruction of Mortal Life, Total.
The entrance to the Lunae Libri is an iron grating that can only be opened with a key. Once the door to the Lunae Libri is opened, rows of torches will light themselves along the sides of the wall. The words domus lunae libri are etched into the stone archway of the entrance below. The steps are cold and mossy, and the air is dank. The stone steps are carved and flat like the floor of an old church. Along either side of the stairs, there are stone boulders, the foundations of an ancient room.
At the bottom of the stairs, in the shadows of the crypt, countless tiny domes curved overhead where the columns jutted up into the vaulted ceiling, forty or fifty in all. Each column is different, some of them are tilted, like crooked old oaks. Their shadows made the circular chamber seem like some kind of quiet, dark forest. The first column is marked with a moon and has a keyhole in it. The first column is from Istanbul. There are columns with the top of the columns, the decorated parts, the capitals from Babylon. A column with four hawk heads poking out from each side from Egypt, and another dramatically carved with a lion's head from Assyria. There are torches along the walls that illuminate the room with flickering light. The stone walls are carved. Some are cut with faces of men, creatures, and birds staring between the forest of columns like predators. Other stones are carved with symbols, hieroglyphs of Casters and unknown cultures. The columns curved around a stone table in the middle of the room. The dusty oak shelves strain under the weight of hundreds of years and centuries of words.