Philip Pullman, The Subtle Knife

This is the second book in Philip Pullman's Dark Materials Trilogy. Lyra, who was the heroine of the first book, and who has left her world (which is similar to but not the same as our world) meets up with Will, a boy from our own world, who has just accidentally killed someone. They meet in a third world, where adults are being attacked by strange presences called Spectres. Lyra uses her alethiometer ( a device which she has intuitively learned to read and which answers her questions) to help her deal with the many challenges she meets. Will becomes the unwilling bearer of a knife with strange powers, and together they struggle to make sense of worlds turned upside down.