I absolutely love Lasky's books, and I am (arguably) a grown woman. This one picks up where the last one left off, and young readers who read all of the Guardians books will get a special treat in this one (no spoilers, though - read the book!)
The thing I like best about these books is how intensely character-driven they are. All of the characters - even Faolin, who in the hands of a less capable author might have been idealized or Mary Sue'd to the point of caricature - change over the course of time, and their growth drives the drama, the suspense, and the plot. This is awesome, because it becomes an opportunity for the parents of kids who like the books to talk about the themes of the book (leadership, trust, self-doubt, stereotypes, religion, spirituality, dogma, differences) on whatever level a particular kid is ready to discuss AND relate it to personal growth.
Fast read for anyone over about 8 years old. Good vocabulary words through sixth grade. Subject matter can be experienced on many levels, making the book interesting for all ages. This is a good series for kids who already like the Erin Hunter and Brian Jacques books. One caveat is that the series raises some complicated social and religious questions. You may wish to discuss these issues with your child, especially if your viewpoint differs.