On special books:
The original artifact is called the "Book of Wisdom" created by Ekidna, Witch of Greed.
The book accesses the World Memory and reveals truth, but it's so powerful that most mortals would fry their brains if they try to read it.
Two less powerful copies of this book exists as white "Book of Gospel", created by Ekidna and each belongs to Roswaal and Beatrice.
Then there's "WItch's Gospel", black books that are poor copies of the original two Gospels, and somewhat reveals events that leads to the future the owner desires. But it reveals in such cryptic ways, it's nearly un-decipherable.
It's practically useless to anyone but the complete insane, and the Bishops of the Cult of the Witch happens to be all insane.
Only the owners can read the book, if anyone else reads it they can't remember what they read.
It's basically the only thing the Bishops listen to. They don't even question it either, they just do it.