His Dark Materials Tome 1
The Golden Compass

Lyra and her demon are 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. She is 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, she'll have to betray the other....