O is for Gregor the Overlander. Chances are you've all read and loved The Hunger Games. This is a series of books aimed at slightly younger readers. Gregor is 11 and falls trough a grate in the laundry room and ends up in the Underland. The Underland is full of bugs, bats, rats and humans and Gregor has to fight to fulfill a prophecy.
This book is really great and full of adventure. I recently gave a copy of this book to a kid I babysit for, we read the first chapter together and then he was hooked.