If the Book of Revelations is used at least once on a floor, a Harbinger will replace the current floor's boss if possible.
This will always happen as long as the floor's respective Harbinger has not been encountered, the boss has not been fought yet, and a Harbinger can appear on the floor normally.
This has a high chance to happen but is not guaranteed.
This item belongs to the Bookworm set. Collecting three items from this set will transform Isaac into Bookworm.
Notes
The Harbingers capable of appearing, and the floors they appear on, are as follows:
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.
This item can replace The Bloat with a Harbinger in Aprils Fool (challenge #32).
Bosses in Downpour / Mines / Mausoleum and their variants cannot be replaced with Harbingers.
Synergies
Book of Virtues: Creates wisps in the inner ring, resembling tiny crosses that do not fire tears. The wisps create a Harbinger that spawns Locusts (the same ones that 7 Seals creates) for the current room when destroyed.
Converter: Allows for generation of Heart Containers. (Schoolbag required)
The Book of Revelation (note the lack of an "s") is the final book of The Bible. It depicts the apocalypse through a vision shown to John of Patmos and includes the Four Horsemen and themes of Hell, Satan, and The Lamb. This explains why using it causes the next boss to be a Harbinger, as well as why it increases the chances of a Devil Room / Angel Room to appear.
Gallery
3DS The icon of Book Of Revelations as seen on the lower 3DS screen.
Book of Revelations's splash image which appears when it's activated.
Soul Heart.
Devil Room or 
Angel Room door to open if held while killing the final boss of the floor.
The chance is increased by 35%.
The chance is increased by 17.5%.
This will always happen as long as the floor's respective Harbinger has not been encountered, the boss has not been fought yet, and a Harbinger can appear on the floor normally.
This has a high chance to happen but is not guaranteed.
This item belongs to the 
Bookworm set. Collecting three items from this set will transform Isaac into Bookworm.
Famine in Chapter 1
Pestilence in Chapter 2
War in Chapter 3, floor 1
Death or 
Conquest (if unlocked) in Chapter 4, floor 1
Headless Horseman on any floor in Chapters 1-4
Mom, 
Isaac, 
The Lamb, etc.) are unaffected by Book of Revelations.
Curse of the Labyrinth is affected.
Boss Rooms, like Double Trouble rooms or if the boss is 
Mr. Fred.
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.
The Bloat with a Harbinger in Aprils Fool (challenge #32).
Bosses in 
Downpour / 
Mines / 
Mausoleum and their variants cannot be replaced with Harbingers.
Book of Virtues: Creates wisps in the inner ring, resembling tiny crosses that do not fire tears. The wisps create a Harbinger that spawns Locusts (the same ones that 
7 Seals creates) for the current room when destroyed.
Converter: Allows for generation of Heart Containers. (
Schoolbag required)
Car Battery: Gives Isaac two soul hearts.
There's Options: Defeating a Boss after using the book at least once in the floor will spawn both a regular Boss pool item and either 
Ball of Bandages or 
Cube of Meat.
Dream Catcher: If the floor boss is revealed to be one of the horsemen, book of revelations can be used early as the downside is no longer relevant.
Genesis: Effects on the 
Satanic Bible: The spawned 
Books 
The Binding of Isaac: Rebirth
