The Diamond of Darkhold is a little like The City of Ember because you have the protagonists, Doon and Lina, racing to solve yet another mystery for the sake of their city. And they are once again trying to read an old document that has been torn apart. Of course, obstacles always stand in their way.
Everyone in the city of Sparks is hungry and overworked. Sparks is a backward place where candles provide the only light at night, where nothing except food is produced, and where there is no electricity or gasoline. To get what they need, they rely on roamers who scavenge items from old houses abandoned hundreds of years ago. In the midst of all of this, Lina and Doon are trying to find a piece of technology left to them by the Builders for when they emerged from Ember. They hope this technology will help Sparks and make their lives a little less hard.
If you enjoyed the earlier Ember books, you will enjoy this one!