The His Dark Materials Series
In a world as convincing and thoroughly realised as Narnia, Earthsea, or Redwall, lives an orphaned ward named Lyra Belacqua, whose carefree life among the scholars at Oxford's Jordan College is shattered by the arrival of two powerful visitors. First, her fearsome uncle, Lord Asriel, appears with evidence of mystery and danger in the far North. He leaves Lyra in the care of Mrs. Coulter, an enigmatic scholar and explorer who offers to give Lyra the attention her uncle has long refused her.
Lyra sets out for the top of the world in search of her kidnapped playmate, Roger, bearing a rare truth-telling instrument known as the Golden Compass. All around her children are disappearing - victims of so-called 'Gobblers' - and being used as subjects in terrible experiments that separate humans from their daemons, creatures that reflect each person's inner being. And somehow, both Lord Asriel and Mrs. Coulter are involved.