Okay... big weird misconception here. First, all the ghosts were ghosts for different reasons. They are just like the Hogwarts ghosts listed in the book. Hogwarts is holding the book because it is our turn to guard it. Marcella found it where it was hidden two terms ago and they decided to brew a potion from it. The book has nothing to do with the ghosts other than it was the means for the potion to be made that brought them back.