The His Dark Materials trilogy by Philip Pullman is often classified as YA, but I didn't read it until adulthood, and I remember thinking that any teenager who read it would certainly grow up to be an incredibly emotionally mature adult. Fantasy is not everyone's cup of tea, myself included, but in the case of His Dark Materials, the fictional universe is very elaborately built up to provide a scaffolding upon which Pullman then dismantles the world in which we live -- and he pulls it off so gracefully. There are two points in the trilogy that stick out to me in particular as being the absolute most I have ever cried while reading a book, and this was more than just a few tears -- I finished the series in sobs; it is also one of my favorite love stories.
The series is often compared to The Chronicles of Narnia by C.S. Lewis, and there are many obvious similarities, including the use of fantasy elements as a kind of stand-in for Christianity. If you found the morality of Narnia to be too simplistic and prefer your religious allegories with heaping doses of cynicism and darkness, then His Dark Materials is more the series for you. The Catholic Church is deeply critical of the series, in part because the characters set out to destroy God; the fact that the human soul exists outside the body in the form of a small animal companion does not help. It is very difficult to read the novels and then accept that you will never have a little "daemon" otter or goldfinch following you wherever you go, and I felt incredibly melancholy after finishing the series because I was so sad to leave this world behind.
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