Northern Lights by Philip Pullman

Lyra and her daemon are eavesdropping when they uncover a plot to murder Lord Asriel. He has returned from the far North where he tells of strange sights-a beautiful city hovering in the sky above the pole, a shimmering Dust that settles on and delineates all but human children. Lyra is scarcely able to save his life before Lord Asriel is off again, on a quest to answer the riddle of the world. Meanwhile, children are disappearing, and when Lyra's dearest friend is taken by the Gobblers, she determines to save him - a decision which leads her on a great journey, a journey that will lead to the discovery of her mother, a disastrous betrayal and the beginning of the unravelling of the mystery of Dust.