His Dark Materials by Philip Pullman

Includes all three novels: Northern Light, The Subtle Knife and The Amber Spyglass.

Northern Lights introduces Lyra, an orphan, who lives in a parallel universe in which science, theology and magic are entwined. Lyra’s search for a kidnapped friend uncovers a sinister plot involving stolen children and turns into a quest to understand a mysterious phenomenon called Dust. In The Subtle Knife she is joined on her journey by Will, a boy who possesses a knife that can cut windows between worlds. As Lyra learns the truth about her parents and her prophesied destiny, the two young people are caught up in a war against celestial powers that ranges across many worlds and leads to a thrilling conclusion in The Amber Spyglass.