The Amber Spyglass - Book 3 - Phillip Pullman

Will is the bearer of the knife. Now, accompanied by angels, his task is to deliver that powerful, dangerous weapon to Lord Asriel - by the command of his dying father. But how can he go looking for Lord Asriel when Lyra is gone? Only with her help can he fathom the myriad plots and intrigues that beset him.