The Library contains a main room with five side chambers, inside one of which is the area's hostage, Father David. The fifth room, which is opposite the area's entrance, contains a projector. Four slides and numerous notes are found in this area, along with a boarded-up door which apparently conceals a Suitor (likely Basile). The room containing the hostage also contains pictures of Justine and her parents.
The puzzle in this room involves using the four slides with the projector and "Puzzle Lock" to open a hidden passage. Failing to do so will result in Father David being ripped apart.
After completing the puzzle, the player will descend into a dark underground room where they will encounter Basile. The Dungeon and Tunnels are accessed from Library, though the door to the Tunnels is jammed.
Each slide found in each room have a different picture of a man: the one with open arms, the one who's kneeling, the one with the sword, and the one being blessed.
Amnesia The Dark Descent: Brennenburg Castle - Rainy Hall - Old Archives - Entrance Hall - Archives - Wine Cellar - Laboratory - Refinery - Cellar Archives - Archive Tunnels - Back Hall - Study - Guest Room - Storage - Machine Room - Elevator - Prison: Southern Block - Prison: Northern Block - Cistern Entrance - Cistern - Morgue - Control Room - Sewer - Nave - Cells - Choir Entrance - Choir - Main Hall - Transept - Chancel - Inner Sanctum - Orb Chamber