The Third Book of Ember, The Prophet of Yonwood, is a prequel, well over 100 years prior to The City of Ember and in the town of Yonwood. A young girl, Nickie, is visiting Yonwood with her Aunt to clean up and sell her great-grandfather's home. Yonwood is the home of a prophet who has seen terrible visions of the future -- war -- and is now in a comatose state. The town is all aflutter trying to follow the visions, trying to follow God, as the verge on fanaticism. They are accusing sinners left and right, and adding new "commandments from God" every time the comatose prophet mumbles something.
This book was okay. It just wasn't what I was really expecting. It was nothing like the other two books, even for a prequel. And to be honest, I found it pretty irrelevant. It covers the space of a few weeks and then in the last chapter it gives a mini epilogue of sorts for the characters. And then, in one sentence, it is linked to Ember. Maybe in reading the fourth book everything will tie together. I'll let you know!