I was a little surprised by this book. Based on the synopsis and reviews I read, I expected to love the story, but upon finishing, I find it stuck firmly in the "okay" section.
Basically, this is "Labyrinth" for young readers, minus David Bowie and his never ending supply of too-tight pants. Girl gets mad at younger brother. Evil creatures from Below/Underground steal brother/soul. Girl follows and spends most of the story meeting strange creatures, learning life lessons and making friends one of whom turns out to be a cowardly traitor, a la Hoggle. Girl also makes liberal use of the phrase "It's not fair," but eventually passes all tests set before her, saves her brother's soul and discovers there's no place like home.
Normally, I would love the dark, weird world of Below, but the book never drew me in. I never cared much about what happened to the protagonist and never worried about her or her brother. Maybe knowing where things were headed dampened its effect.
Whatever the reason, I would still recommend this book to readers looking for a little creepy in their reading diets.