The fourth book in a series of five. Twelve-year-old Gregor and his three-year-old sister Boots return to the Underland and help Queen Luxa find out why all the mice have suddenly disappeared. This book tackles some difficult questions, such as when war is appropriate, genocide, and how to deal with an unstable power-mad leader. Creative, thoughtful, well-written — I like this series very much. For grades 4-6.