Tell Alison to Go check it out!
You hesitate a moment before you start walking towards the ghost, Allison's hand, in yours. You're not even entirely sure
why you took her hand; Allison doesn't seem to be worried, or foolish enough to run off. You just... feel better this way.
As you get closer, the ghost beckons, then points to a specific book on the bookshelf. As you watch, it tries to pull the book away from the shelf, but its transparent finger just drifts right through the book. The ghost tries again, then gestures to you.
That's... strange. Several of the other ghosts had books. Transparent books yes, ghostly like them, but you assumed the books came from the shelves. Maybe it wants you to pull the book out for it, so it can read, or something...
I say we go over and see what he's trying to show us.
Well... you don't really see how this could be a trap. You reach for the book, keeping your eyes on the ghost just in case it tries something...
The book is difficult to pull free. It's almost like it's being held there, squeezed against the edge of the shelf, even though it's clearly not. You have to pull surprisingly hard to make it come free--and then suddenly the resistance is gone, and the book is in your hands. You start to look at it, the cover blank, without any markings.
Then you jump, the stone wall in front of you making a strange high-pitched noise, like a long note from an instrument falling into a chasm. A square of stone pushes away from the wall, then slides upward, the sound growing quieter as it rises. And then it stops, a dark tunnel left behind.
The ghost turns to you and gestures oddly, bowing a little and touching its temple with a finger, then walks briskly towards the new passage. You nervously grip the book you took with your free hand, your other one still gripping Allison's hand. You feel shaken by the sudden sound of the lifting stone, but Allison is unperturbed, waving to the ghost as it disappears into the passageway.
...Why did the ghost need you to open the way? Can't ghosts walk through walls?