The Spookipedia is a mystical artifact that contains an encyclopedia of knowledge in regards to spirit objects and Frightside/Demon World's history. It originally belonged to Enma's son, Koenma, but was then passed down to Scary Godmother sometime prior to the first film.
The book can identify and locate a spirit object, even if said object is in a different dimension.
The only known time it was ever used was in Movie 3, where Scary Godmother used it in order to locate the Vile Vial. It's possible that she has been in possession of the book for much longer (as it is implied in the manga adaptation), but it is unclear why Koenma had chosen her as it's new owner.