The Underland Chronicles #1: Gregor the Overlander, by Suzanne Collins, author of the Hunger Games.
I confess that I didn't realize that Ms. Collins had written a series before The Hunger Games trilogy. The first book in The Underland Chronicles, Gregor the Overlander tells the story of a young boy who falls into a world below New York City and sets out on a quest to find his father and to save the new world he has discovered.
Age level: 9 and up (per the book's product page.)
Gregor the Overlander received 156 five star reviews out of 261, for an overall average of 4.3 stars.
330 pages; text-to-speech, X-ray and lending enabled.