The Amber Spyglass by Philip Pullman (2000, Hardcover)

Lyra has been taken by her mother, Marisa Coulter, to a remote cave in the Himalayas of their own world, where she is kept in a drugged sleep. In this state, Lyra dreams that she is in a wasteland talking to her dead friend Roger, whom she promises to help. In Cittàgazze, a pair of angels named Balthamos and Baruch tell Will that they have come to take him, the bearer of the subtle knife, to Lord Asriel. Will refuses to go until Lyra is rescued, to which the two assent. Will & the angels are attacked by a soldier of the archangel Metatron, the Authority's Regent. Will cuts a window into another world to escape. A sect of the Magisterium called the Consistorial Court of Discipline learns where Lyra, considered the next Eve, is being held. They send out a small army to capture & kill her. They also know that a woman, Mary Malone, is fated to play the role of the serpent, tempting Lyra; therefore they send a priest named Father Gomez to follow Mary and subsequently kill Lyra. Lord Asriel sends out a small army to Lyra's cave, to counteract the zeppelins from the Consistorial Court. He also sends two Gallivespian spies, the Chevalier Tialys & the Lady Salmakia, to protect Lyra. Gallivespians resemble humans, but are approximately four inches tall. Mary Malone, who has stepped through a window from the reader's world into Cittàgazze, eventually goes through another window into a stranger world. She finds a group of elephantine creatures who call themselves mulefa & travel by attaching round seedpods to their feet & using them as wheels. These creatures have a complex culture, intricate language & an infectious laugh; as a result, Mary begins to think of them as people. Eventually, Mary is absorbed into mulefa community, where she learns that the trees from which the seedpods are gathered are becoming extinct, & have been so for 300 years. Mary, to further understand this problem constructs a telescope out of sap lacquer that allows her to see Dust. It seems to be flying off into the distance in large streams, rather than falling downward & nourishing the trees on which the mulefa mutually depend. Will meets Iorek Byrnison, the king of the armored Panserbjørnes, whose people are migrating south to avoid the Arctic melt caused by global warming. Iorek agrees to help rescue Lyra. Here, global warming is associated with similar disasters taking place throughout many worlds as a result of the upheavals & the Church's army--converge on Mrs. Coulter's cave. Will is able to wake Lyra. He is cutting a window into another world when Mrs. Coulter turns & looks directly at him. For a moment, Will is reminded of his own mother. His concentration falters, and the knife shatters. Because the window he has cut is open, Will, Lyra, & the Gallivespian spies manage to escape to another world. Will & Lyra enter the world of the mulefa. They grab their dæmons immediately before doing so; but the dæmons run off again. Lyra & Will's dæmons return & tell them that all the windows between the worlds must be closed, as Dust is leaking out of them all the time. Furthermore, every time a window is made, a Spectre is created; therefore the knife must be destroyed altogether. The angels will allow one window to remain open: the one leading out of the land of the dead. Because Will & Lyra have fallen in true love with each other upon the onset of puberty, & because they cannot live together, this information deeply saddens them.