Gregor and the marks of secret - Suzanne Collins

The fourth book in a gripping young fantasy series from the author
of THE HUNGER GAMES.
It's only been a few months since Gregor and Boots returned from
the Underland, leaving their mother behind to heal from the plague.
Though Gregor's family receives frequent updates on her condition,
they all know Gregor must return to fulfill 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
- 12-year-old Luxa - and Ripred the rat to defend the Underlanders
and the vulnerable "Nibblers," or mice, from the rat army.