Important information: Every book of this series is written by a different author. Brandon Mull was the first and my favorite.
The plot of this story was good. It was exciting and, while not the most original book ever, it made you want to know what happens next. The ending, though, wasn't exciting. I love (and hate) a good, dramatic cliffhanger that just makes you die waiting for the next book. The ending here wasn't one of those. Like, I'll probably read the next book, but I'm not particularly compelled to.
Now, the characters. This author did it better than the last one. The normal main characters haven't really changed or grown too much, but the addition of Worthy was interesting in a good way. He has grown and changed tremendously.
The setting is well-done as usual. Erdas has some complex history, and it's fun to learn more about, being explained understandably. It does seem like the authors add things to it on a whim, but it isn't terribly confusing.
I spotted two grammatical mistakes: A period was used for a question, and woman was used for the plural form. "The woman...their heads." Other than those, I have no technicalities to pick on.
To sum it up, the plot and setting were good and the characters were okay, but the ending wasn't good and there were a couple grammar mistakes.