Gregor and the Marks of Secret, Paperback/Suzanne Collins

It's only a few months since Gregor and Boots returned from the Underland. Now Gregor must return to fulfil his role as the warrior who is key to the Underlanders' survival. Accompanied by his now-talkative little sister Boots, still considered the honorary "princess," Gregor joins forces with another princess to defend the Underlanders.