The Book of Riddles is a book found by Luigi in the Study of the mansion. One riddle can be read by Luigi. It describes how if one keeps away from a mystery, they should go back and look at it from a different point of view. This is a great way to help Luigi defeat Neville, as it is provides a hint to capturing him.
"Mysteries appear wherever people turn... ...but turning away from them solves nothing. Instead of turning away, turn to the side and look at the mystery from a different view. A way will surely open."