The Prophet of Yonwood

The Book of Ember is quite a lovely imagining.

This one goes back in time and fills in some of the world, but I did not find it as well done.
I did not feel as connected to the characters or the story. It seemed not quite real even when set in a more real world. There was so much potential, too - a bit of cultish behavior, some interesting science, political upheaval, etc.

You don't have to read this one to enjoy the rest of the series.