The Golden Compass by Philip Pullman

Lyra is rushing to the cold, far North, where witch clans and armored bears rule. North, where the Gobblers take the children they steal--including her friend Roger. North, where her fearsome uncle Asriel is trying to build a bridge to a parallel world. Can one small girl make a difference in such great and terrible endeavors? This is Lyra: a savage, a schemer, a liar, and as fierce and true a champion as Roger or Asriel could want--but what Lyra doesn't know is that to help one of them will be to betray the other.