I don't read as much middle grade as I do YA, since that's what I write, but I thoroughly enjoyed Brandon Mull's Sky Raiders (and I think my nine-year-old will love it).
Twelve-year-old Cole Randolph just wanted to impress his friends and the girl he liked by telling them about the spooky haunted house set up near by. But they got much more than they bargained for, when the haunted house turned out to be a set up by slavers from the Five Kingdoms. Cole manages to hide, but when creepy grownups start pulling his friends through a hole in the floor, he can't abandon his best friend and the girl he likes. So he follows them--only to find himself in an alternate world. He's quickly taken captive and sold to Sky Raiders, an unusual salvage operation who try to salvage goods from weird sky castles that float from the Eastern Cloud Wall to the Western Cloud wall. No one knows where they originate--they don't seem to exist past the cloud walls--and while sometimes they can be rich treasure troves, other times the castles are deadly. Most new slaves don't survive the fifty minimum missions to rise through the ranks of the Sky Raiders.
But when a stranger comes for the girl Mira who works for the Sky Raiders, Cole finds his own mission to save his friends temporarily hijacked by his need to save a new friend.
Like most of Mull's books, the most fascinating part here was the world he's created. The book is the start of a five-book series, one for each of the Five Kingdoms, which practice their own form of magic. My favorite part of the book was his time with the Sky Raiders, but all of it was interesting and quick-paced. If I have any complaint, it's that sometimes the characters themselves take second seat to the action--aside from being brave, I don't feel like I know a lot about who Cole is. But the story is such a fun read, I'm not sure that matters.