I've really enjoyed the unique story of this trilogy, but I have to say, the last book was a bit of a disappointment.
Chloe, Tori, Derek and Simon have just escaped the Edison Group again and are hiding out with the group fighting against the Edison Group. Once again, a large part of the beginning of this book is spend doing very little. The action comes all at once, at the end, and involves a lot of twists, turns, and constantly flipping allegiances (Are these people really on our side? No! Let's run away! Wait, maybe they are. Let's stick around. Wait! We have to run away!)
Mostly, this book did not feel like it should be the end. The ending is pretty ambiguous as far as endings go, and many, many aspects of the plot were left unexplained. Some things were brought up in the storyline of this book only to be dropped casually a few chapters later. If this series was slated to continue in additional books, I would assume these things would come up in the next book, but as this is the final book, it seemed like the plot wasn't as well thought-out as it should have been.
On the positive side, Armstrong does a pretty good job of subtly reminding the reader what has happened previously. I didn't remember many details from the previous books, but whenever I'd start to be confused, there were some helpful hints placed into the narrative that reminded me of what I was supposed to know. Still, this is the 3rd book, and doesn't stand alone. If you're gonna read the series, start with The Summoning.