The Prophet of Yonwoodtakes place in the time before the City of Ember even existed. The world is facing wars and imminent destruction, when a young girl visits the town of Yonwood. A respected woman in the community has a vision predicting the destruction of the world.
I thought this prequel was disjointed and simply strange. It didn't seem to have a cohesive storyline and wasn't as well-crafted as her other two books set in this world (see reviews here and here). I was actually pretty disappointed.