Darkness is Their Cheese! is a book found by Luigi in the Study of the mansion. A passage from it is written by C. Parmesan. He is possibly a human who visited the mansion before Luigi. His passage describes Ghost Mice liking darkness and real mice liking cheese. He tells the reader to find ghost mice from the darkness in their rooms before the lights are back on.
"Ghost mice are drawn to darkness as real mice are drawn to cheese... In the dark, they shine gold with contentment. I must try to get close to one before the lights come back on. (C. Parmesan)"