Five Kingdoms #1: Sky Raiders by Brandon Mull

Cole and his friends visit a haunted house on Halloween, but they're quickly kidnapped and taken to another world, the Outskirts. On the way to the High King, Cole is sold as a slave to a gang that is salvaging treasures from floating castles. He becomes a scout, and his dangerous job is to explore castles to discover their possible treasures and dangers. Most scouts are expected to die, and Cole is lucky to survive his encounter with a huge scorpion-like monster. His next mission seems to be a little safer, until the gang is attacked by a cyclops, giant snake, and other creatures. Cole is left behind with a girl named Mira, and she reveals a secret about the evil High King now ruling the five kingdoms. They manage to escape the floating castle, but their lives are in even greater peril when four hundred of the High King's soldiers show up. Then there's the giant monster ravaging through the kingdom with a connection to Mira...Who is she?

The author also wrote the Fablehaven series, and I've enjoyed his work in the past. The plot includes quite a bit of adventure and enough action to keep most readers interested; there's a little bit of a lull about two-thirds of the way into the book. There's also an element of mystery surrounding Mira, and Cole has been told there's no way to get back to his real world. Cole's character shows a great compassion for others, and his bravery is realistic. There's no sense in doing something heroic for no good reason.