4th&up. 
 Before she received widespread public attention for The Hunger Games Trilogy, Suzanne Collins wrote another amazing fantasy series called the Underland Chronicles. I just reread the first book- Gregor the Overlander- and it was every bit as good as I remembered. 
 With an absent father, an overworked and stressed mother, not enough money and too many responsibilities, eleven-year-old Gregor has a life he wouldn't mind escaping from, but he doesn't expect to escape to another world! That's just what happens, though, when he and his baby sister fall through a laundry room air duct and into a subterranean world called Underland. Populated by huge bats, rats, cockroaches, and spiders, as well as people unlike any he's ever known, Gregor finds himself caught up in the adventure of his life. 
 I highly recommend this wonderful fantasy/adventure series.