It's a bookshlef you store books in.
The Bookshelf is located in the Glass display room on the 2nd floor.
You put the Book of Death in an empty spot on the shelf inorder to unlock the glass case with the butterfly model in it.
In Extra mode it is not used for The Book of Death instead if you put The Book of Death the shelf will crush you. To avoid that you have to instead put The Book of Death on the secret bookshelf loacted in the same room. After you have done that a Bloody Skull will chase you. Run to the Bookshelf and run around it starting from behind. If you start from the front the Bookshelf will crush you again. Once you have run behind the Bookshelf it will crush the Bloody Skull but proceed running out of the room since it will once again chase you.
If you backtrack in the game and walk into the Glass display room and go to the shelf you will see that it's standing up again, if you walk infront of it it will crush you.