It's not that I didn't like this one. In some ways, it's as good or better than the first three and deals with some pretty mature themes (genocide, species-ism, oppression). But it's the first of the Underland chronicles to suffer from series-itis. There isn't enough of a resolvable story arc. I hope the next, and final, installment redeems it, but I really like for each book in a series to at least plausibly stand on its own. And these had been doing it so well!