Library

The Library is a room introduced in the Wrath of the Lamb. It can appear on any floor. It contains two books, chosen at random. The room around you contains bookshelves, and the floor is similar to that of the Cellar's. It can contain any book in the entire game except The Bible.

The Library has a 5% chance of spawning, and a 25% chance of spawning if you have touched a book that is part of the pool in the run.

Notes

The Library can contain two of the same book; the charge of each will be equal and linked; that is, using one will use up the other's charge as well.

From outside, the Library's door is identical to that of the Shop's, making the two impossible to distinguish when under the Curse of Darkness.
Because picking up an item means it cannot ever be seen again in the same run, picking up every book that can be found in the Library will force the Library to spawn standard Treasure Room items. This can be easily abused with a combination of The Dice and the Habit.
The Library will not spawn the The Book Of Belial if you play as Judas. Holding the Book of Belial as Judas does not provide an increased chance of finding a Library.
The Library will not spawn The Monster Manuel until it is unlocked.
Despite being a book, The Bible is not part of the normal Library item pool.

List of Books in The Library

Item	Icon	Effect
Anarchist Cookbook		Spawns six troll bombs around the room.
The Book Of Belial		Adds 2 points of damage for the current room. Damage is further multiplied by 1.5x with the Blood Of The Martyr item. Also increases chances of a Devil Room spawning by 25%.
Book Of Revelations		Adds 1 Soul Heart and makes the next boss a Horseman (except on XL Floors and on floors with fixed bosses such as Mom). Increases chances of a Devil Room spawning by 35%.
Book Of Shadows		Grants Isaac invulnerability for 10 seconds.
The Book Of Sin		Spawns a random Pick Up, Pill or Card.
Monster Manuel		Summons a random familiar for the current room only, even if it is locked.
The Necronomicon		Damages all enemies in the room significantly.
Telepathy For Dummies		Gives Isaac homing tears for the current room. (Identical to Spoon Bender)