Philip Pullman doesn't underestimate kids. He trusts younger readers to think about really tough stuff: the loss of innocence, original sin, and the question of free will. Sure he writes books for kids, but the His Dark Materials trilogy is often considered Literature with a capital L rather than belonging strictly to the young adult or children's genre.
But wait, does that mean this book is hard and boring and will put you to sleep? Um, no. This is a fantasy novel, after all. Some of the themes are challenging, but there's plenty of magic and action to keep you reading – and thinking.