Book of Revelations

Effects

Gives one (Soul Heart) Soul Heart.
Increases chance for a (Devil Room) Devil Room or (Angel Room) Angel Room door to open by 17.5% ((Removed in Afterbirth)35%) if held while killing the final boss of the floor.
If the Book of Revelations is used at least once on a floor, a Harbinger will (Added in Afterbirth / Removed in Repentance) sometimes/(Added in Repentance) always replace the current floor's boss if possible.
(Added in Afterbirth †) This item belongs to the Bookworm set. Collecting three items from this set will transform Isaac into Bookworm.

The Harbingers capable of appearing, and the floors they appear on, are as follows:

Set bosses ( (Mom) Mom, (Isaac) Isaac, (The Lamb) The Lamb, etc.) are unaffected by Book of Revelations.
Only the first boss of a floor affected by (Curse of the Labyrinth.png) Curse of the Labyrinth is affected.
Book of Revelations won't replace the boss on double-sized (Boss Room) Boss Rooms, like Double Trouble rooms or if the boss is (Mr. Fred) Mr. Fred.
- Furthermore, if the Book of Revelations is used on the first floor of a Chapter with a Double Trouble room no Harbinger will spawn on the second floor, even if Book of Revelations is used again. If finding a Harbinger is desired, Isaac should wait until seeing the shape of the Boss Room before using it.
Keeping Book of Revelations until Chapter 4 is one of the methods to unlock achievements related to Super Meat Boy and Super Bandage Girl, explained in detail here.
This item can replace the Boss even after Isaac initiates a battle with a normal boss and then teleports out before the fight is completed.
(Added in Afterbirth † / Removed in Repentance) This item can ((Added in Repentance) will) replace (The Bloat) The Bloat with a Harbinger in the Aprils Fool challenge.
(Added in Repentance) Bosses in (Downpour) Downpour / (Mines) Mines / (Mausoleum) Mausoleum and their variants cannot be replaced with Harbingers.
(Added in Repentance) This item will replace (Mother) Mother with Death or Conquest during the Red Redemption challenge.

(Added in Repentance) Book of Virtues: Creates wisps in the inner ring, resembling tiny crosses that do not fire tears. The wisps create Harbinger Locusts (the same ones that 7 Seals create) for the current room when destroyed.