Gregor the Overlander
by Susanne Collins

Melissa says,

Gregor the Overlander is the first of five books in the Underland Chronicles, which Suzanne Collins wrote before The Hunger Games.

Gregor and his little sister Boots are trapped in the Underland in the midst of a long-standing war between humans and rats. It’s a wonderful story of adventure, peril, and family loyalty with unexpected allies and betrayals.