The first two books were brilliant, completely addictive reading. This book seems to sit in the middle of nowhere (even though it is the prequel to The City of Ember) it really only steers the plot towards Ember in the last chapter, giving you very few clues about how Ember came to be built. I was disappointed (but it would be difficult to keep up the excitement without a break!)
Must be read if you are reading the Ember series as it's another year to wait until the last book, but don't expect find much about Ember.