His Dark Materials The Subtle Knife by Philip Pullman

The provided text introduces Will, a twelve-year-old boy caring for his troubled mother, who becomes entangled in a dangerous search involving a mysterious green leather case sought by unknown men. Fleeing into another world via a hidden window, Will encounters Lyra, a girl searching for answers about a substance called Dust, and they navigate a city haunted by soul-consuming Specters where adults have vanished. Simultaneously, in the Arctic, Lee Scoresby, an aeronaut, seeks out Dr. Stanislaus Grumman, a shaman with knowledge of other worlds and prophecies, while witches like Serafina Pekkala and Ruta Skadi investigate the disturbances between worlds and the machinations of Lord Asriel, who is amassing forces for a cosmic war against the Authority, and Mrs. Coulter, who seeks to control a powerful artifact called the subtle knife. Will's journey is tied to this conflict, as he comes into possession of the subtle knife and is told it is crucial to Lord Asriel's plans, guided by angels and pursued by those who also desire the knife's power, including the manipulative Sir Charles and the determined Mrs. Coulter.