Book-window
From MYSTlore
The Book-windows were made of a special substance, which Catherine wrote in Tay. Placing one on a blank linking panel of an unpowered book, would make it visible and functional, without requirement of external power, like Gehn's Fire Marble domes.
The linking books used by the Moiety were nonfunctional books stolen from Gehn and made use of that devide. The secret linking book of Tay has such a stone, as well as the linking book back to Riven, held by the Gehn idol in the Tay cave.
Also, when Nelah came to the Stranger for a second time, she brought a Book back to Riven and placed a stone on it for him to link back to the slab room.
