The Underland Chronicles 4 and 5

It didn't take me long to finish the last two books - Suzanne Collins is a master at writing cliffhangers. Book 4 and 5 are more like one book than the previous three, and differ vastly in tone as well. Collins ramps up the violence as the Underland humans and rats prepare for war, transforming the series from mere adventure books.

( Mild spoilers )