Mercutio lets off a half-dozen random quips, idling flitting through the books. Hefting them, he randomly finds one that is significantly lighter than the others. He shakes it, and something thumps on the inside. Opening it, he finds a hollowed-out book with a curved piece of rectangular crystal inside. It looks like the exact size and shape of the hollow section of the crystal ball in the second room.
The magically inclined duo both examine the aura, comparing notes on their studies. The inquisitor has a more practical approach to magic, while the enchantress brings a deeper insight into the esoterica of arcane theory. After a few minutes they come to the conclusion that this must be some sort of dormant teleportation spell, with some sort of trigger device integrated. Likely the trigger is connected to the coin-sized hole in the wall. Perhaps there is some sort of item that is keyed to the spell, allowing passage.