"Northern Lights", the first part of the "His Dark Materials" trilogy. I have had this book recommended to me many times, and I love Pullman's writing, ever since "The Good Man Jesus and the Scoundrel Christ", but I've only just got down to actually reading it. Pullman creates a strangely alien-yet-familiar universe in which people have daemons, animaliform companions, roughly analogous to a soul - a core part of their being. The book was made into a movie, "The Golden Compass", which although it apparently did OK at the box office, wasn't supposed to have been great.
Anyway, forget the movie - just read the book. It is absolutely fantastic, as Lyra Belacqua (the wild-child heroine) and her daemon Pantalaimon race across an alternative Northern Europe to confront an enormous evil and to uncover the mysteries of Dust. I'm just about to start the sequel, "The Subtle Knife", and am looking forward to it enormously. It's all there - loyalty, intrigue, betrayal, philosophy, religion, heresy. A real gem.