It is not required to beat the game, but like the Teddy, it can be used to reveal a room.
The Book's main usage is to unlock the Slendrina's mom easter egg. This is done when the Book is placed on the lectern 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 - Wardrobe
- 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.
- But when the player opens the Book up in the Bookshelf Room, it is empty.
- Granny may have taken the book and released Slendrina, because in Slendrina X, the protagonist leaves the castle without taking the book. This also explains why there is nothing in the book.
- 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 or when the Bloody Mannequin is knocked over.
- This book was used in a ritual to seal or release Slendrina.