Book of Revelations

Book of Revelations is an unlockable activated item.

Effects[]

Gives one Soul Heart.
Increases chance for a Devil Room or Angel Room door to open by 17.5% (35%) if held while killing the final Boss of the floor.
If the Book of Revelations is used at least once in a floor, a Harbinger will replace the current floor's boss (all other Boss generation rules are still respected):

The chance is increased, but not guaranteed.
This item belongs to the Bookworm set. Collecting three items from this set will transform Isaac into Bookworm.

Set bosses (Mom, Mom's Heart, It Lives, Isaac, ???, Satan, and The Lamb) cannot be replaced by the Book of Revelations.
Only the first boss of an XL floor is affected.
Book of Revelations won't replace the boss on double-sized Boss Rooms, like Double Trouble rooms or if the boss is 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.
Bosses in Downpour / Mines / Mausoleum and their variants cannot be replaced with Harbingers.
This item can ( will) replace The Bloat with a Harbinger in the Aprils Fool challenge.
This item will replace Mother with Death or Conquest during Red Redemption challenge.

Book of Virtues: Spawns wisps in the inner ring, looking like 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.

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.