Collins, Susan, Gregor the Overlander,
First book in the series. Gregor and his sister Boots fall into the Underland through the laudry room. He is the warrior who must save the Underland from the rats. He encounters roaches, bats, rats, spiders, and humans. He goes on a quest with all of the above to save his father who had fallen int the Underland two years before. good plot and action.