The fourth book written, you're nevertheless the first chronologically. You not only describe the creation of Narnia and tell where the White Witch, the lampost and the wardrobe came from, you get to bounce between worlds with the help of Uncle Andrew's weird magic rings.
I'm not even sure that I read the Narnia books. I did have more of a fondness for Lloyd Alexander's "Black Cauldron" series, and somehow, never got into Narnia as much.