Book four of the Underland Chronicles deals with some pretty dark stuff. It picks up the story not long after the end of book three, with Gregor's mom still recovering in Regalia, and Gregor unwillingly continuing his echolocation lessons with Ripred the Rat.
Gregor and Boots are regular visitors down below, but things take a bad turn when Luxa receives a distress signal from the nibblers while the friends are enjoying a birthday party for their friend Hazard. Gregor and Luxa try to slip away undetected to investigate, but their plans are foiled and the expedition is forced to become a group picnic, complete with children and the ever present shrimp salad.
Gregor is relieved that for this adventure at least he is not under pressure as the key figure of a prophecy, but learns that things are not always as they seem. He encounters the fast growing Bane, and is deeply troubled by the Hitler-like aspirations of the white rat - but even more troubling is his inability to master either echolocation or the rage that transforms him into a warrior.
Although still written in age-appropriate language, within this book lies violence, death, erupting volcanoes, poison gas, deadly creatures, the genocide of a species, and the outset of war. In addition, like that other embattled hero, Harry Potter, Gregor's hormones are beginning to warm up with similarly awkward results.
To fully appreciate this book, you'd be better off reading books one through three first, but be warned that the ending of this one tantalizingly sets the stage for the final installment "Code of Claw", which unfortunately isn't expected until 2007.