Temple Of The Four (Dune Quest 1-8)

Synopsis
The Temple of the Four kept many mysteries. Nobody except the monks knew how to open its doors. And only the monks knew where the grotto with the Order's treasures could be found. It meant that the Indistinguishable could be right: one of the monks cold have taken the book.

Completion
How can we find out which monk has the Book of the Empire? It feels like I'm the protagonist of a cheap detective story.