It is not required to beat the game, but like the Teddy, it can be used to reveal an interesting easter egg.
The Book's main usage is to unlock the Slendrina's mom easter egg. This is done when the Book is placed on the stand in the Bookshelf Room. When done so, Slendrina's mom will appear hanging by her wrists in the room.
The Book can spawn in a variety of different locations (for more information on this see Item Positions).
These positions are:
- Dining Room - Shelf
- Dining Room - Glass Cupboard
- Study - Glass Cupboard
- Secret Area Top Floor - Below Drawers
- Garage - Middle Shelf, Near Garage Door
- The Book looks very similar to the books that can be found on the Bookshelf
- The Book, though it is empty, could be a reference to Slendrina X, where the player uses this exact Book to seal Slendrina's spirit inside.
- It's also the same book seen in Slendrina: The Cellar.
- When dropped, it makes a soft noise which is very similar to when the cardboard Boxes are knocked over on the Secret Area Top Floor.