The Book of Belial

Effects

Gives +2 damage for the current room.
(Removed in Afterbirth) Guarantees the (Devil Room) Devil Room or (Angel Room) Angel Room to spawn after killing a boss while holding it.
- (Added in Afterbirth) Increases the chance to open the (Devil Room) Devil Room or (Angel Room) Angel Room by 12.5%.
(Added in Afterbirth †) This item belongs to the Bookworm set. Collecting three items from this set will transform Isaac into Bookworm.

(Judas) Judas starts with this item by default.
(Removed in Repentance) The damage bonus does not stack with multiple uses in the same room or with (XV - The Devil) XV - The Devil.
- (Added in Repentance) Additional uses in the same room or combined with (XV - The Devil) XV - The Devil give +1 damage per use.
While activated, switching items will end the bonus damage effect.
If the (Added in Afterbirth †) Schoolbag and a second activated item are held, the book must be equipped in the primary slot to receive the (Devil Room) Devil Room / (Angel Room) Angel Room bonus.
(Removed in Afterbirth) Only guarantees Devil/Angel Rooms on floors that they can normally appear on, and, unlike Goat Head, picking up The Book of Belial after killing the boss will not open the Devil/Angel Room door.

Blood of the Martyr: Causes The Book of Belial to apply a 1.5x damage multiplier on top of its normal effect. This multiplier cannot stack with multipliers from Cricket's Head or Magic Mushroom.
(Added in Repentance) Book of Virtues: Spawns black and red wisps in the middle ring. Wisps deal double damage compared to normal wisps.
(Added in Repentance) Car Battery: Gives +3 damage for the current room.