But for anyone with a sense of adventure and a desire to learn more about the background of Narnia, I would highly recommend "the Magician's Nephew."
Just as "the Lion, the Witch, and the Wardrobe" contains Christian symbolism (specifically, the atonement of Christ), "the Magician's Nephew" contains symbolism about the Creation. All of the books in the series have children as main characters, which I love because of their perspective, and I think C.S. Lewis nails the dialogue and personalities. It's been great to read aloud.
But you don't have to take my word for it!