The fourth in DuPrau’s Ember series.  I very much enjoyed the first book, The City of Ember.  I can’t remember much about the second two books.  In The Diamond of Darkhold, Lina and Doon are back, this time searching for a way to make life easier for the people of Sparks.  They come across an old book that seems to be leading them back underground, to the city of Ember, where they will find a treasure to bring back to their new city.  A great book until the last chapter.  There, DuPrau apparently wants to quickly wrap up this series, so she writes a nice neat fairy tale ending for everyone, while adding to a random subplot that is just weird.  If you read this book, just stop at Chapter 26 and you’ll enjoy it so much more!