The Subtle Knife
The main setting is a universe dominated by the Magisterium, an international theocracy which actively suppresses heresy. In Lyra's world, humans' souls naturally exist outside of their bodies in the form of sentient "dæmons" in animal form which accompany, aid, and comfort their humans. An important plot device is the alethiometer, a truth-telling symbol reader. By setting the alethiometer's hands to point to symbols around a dial a skilled practitioner can pose questions, which are answered by the movement round the dial of a further hand. The Subtle Knife of the book's title is a knife that is capable of cutting windows between worlds.