The Amber Spyglass by Philip Pullman

The Amber Spyglass is the third and final novel in the His Dark Materials series, written by English novelist Philip Pullman, and published in 2000.
The Amber Spyglass won the 2001 Whitbread Book of the Year award, a British literature award, making it the first children's novel to receive the honour. The Amber Spyglass was named Children's Book of the Year at the 2001 British Book Awards, and The Amber Spyglass was also longlisted for the Man Booker Prize, again the first time this had happened to a children's book.
At the end of The Subtle Knife, Marisa Coulter captured Lyra. She has now relocated her to a remote cave to hide her from the Magisterium, who are determined to kill Lyra before she yields to original sin. In order to keep her hidden, Marisa forces Lyra to drink drugged tea that puts her to sleep. While deeply asleep, Lyra dreams that she is in a wasteland (later realized as the land of the dead) talking to her deceased friend Roger Parslow, whom she promises to help.
In Cittàgazze, two angels, Balthamos and Baruch, tell Will, the bearer of the Subtle Knife, that they are taking him to Lord Asriel. Will refuses to go until Lyra is rescued, to which the two assent. However, they are attacked by a soldier of the archangel Metatron, and Will uses the knife to cut a window into another world to escape. Baruch flies to Lord Asriel to tell him what has happened and to get help.
Meanwhile, an assassin is dispatched from the Magisterium, as they have determined that Dr. Mary Malone is the "Tempter"... (Summary by Wikipedia)