The best part about The MirrorMasters is the main character, Leah. The author does a wonderful job of making her a complex character in which there is no question in what she considers important to her, how much she values her family and friends and how she will admit to being scared to learn the vital role she plays in her quest, yet shows courage in the face of that fear.
The world building is also strong, both in terms of the city that Leah calls home on Earth and the distant planet she is actually from. Both are distinct environments with something unique about them and give the reader an idea about the place where Leah grew up and the new planet she experiences for the first time in many years.
Though I liked how the plot developed, my one problem with it was how it was resolved too quickly. The climax came about rather abruptly and there wasn't much time after the resolution for any real reflection on the events.
I still consider the book a recommended read, however, because the character development and world building are done so well.