SPOILER: You have to find a secret wall, walk through it, and come in from the room to the left! Search for a wall that looks different.
Ok, I will try that soon. A wall that is different is pretty suspicious. I did find in the game some secret entries where you can walk where you cannot to find secret pathways that skip time to go there. I wonder if that is the same with the room...
(Also I found a game similar to this called "WHO STOLE THE BOOKS?"