The Book of the Dead is similar to the Udjat Eye, in that it helps you locate a secret level exit to take you to bonus stages.
It will only open when the Spelunker has the Book of the Dead in their inventory.
The Book of the Dead can be used to find the location of the door before it comes into view.
When the Spelunker nears the spot directly above the door, the cover of the Book will start to become agitated. The spot at which the face on the book writhes the fastest is that directly in line with the door.
To access the door, the Spelunker must defeat Olmec by causing it to fall into the lava right below the door. He will have to break through ten layers of bricks to create a hole deep enough for the Boss to fall through, and then stand on top of Olmec as it sinks into the lava.
This is the only way to enter the door, as there is no ledge below it and cannot be reached without something to stand on. However, it may also be possible in certain circumstances to push a pair of sliding blocks into the lava to use as a ledge, although this relies on the random level generation to be possible.
Once the player enters Hell, the book has no practical function. However, the book will continue writhing faster the deeper the player travels downwards, writhing fastest inside of Yama's Throne .