So mad at Riven endgame

So I got all the way through to the Power/Fire crystals and got to Ghen, but of course the bastard is suspicious of the trap book and won’t use it. So I go back to Riven and scrape my brain for hours in what I could possibly be missing. Finally I crack and look up a hint in UHS and it says that I need to remember some line from the introductory cut-scene that the book will only hold one prisoner at a time. Are you kidding me!!? How are you supposed to be expected to remember that line from 20 hours of gameplay (and weeks of actual time) previous!?

I have loved the puzzling in this game for the most part, but this one is just unfair.