I borrowed the whole series from dwin, and I've just finished reading it. I've heard from somewhere that it's a Christian story. I treat it as a work of fiction, but there are certainly many parallels between the stories and the Bible.
The first book describes the creation of the world of Narnia by a supreme being, the lion Aslan. Similar to biblical creation, first the world was created, then the animals. There is no mention of evolution. An evil is introduced into the world in the form of a wicked witch. The boy who brought the witch in has to make amends by taking an apple from a tree in the centre of a garden. He is not supposed to eat the apple. The description of the place makes it sound like Eden. However, the witch is already there and tries to tempt him to eat it. Unlike Eve, the boy does not succumb, although he came close to doing so. Sounds like the serpent tempting Eve in Genesis 3.
The second book in the series is "The Lion, the Witch and the Wardrobe". It was the book that was made into a movie. The most notable parallel would be the killing of Aslan by the witch, and his subsequent resurrection. Aslan had offered himself to the witch in exchange for the life of one of the children, who had been a traitor but subsequently repented. Aslan himself had done no wrong, but paid for the misdeeds of the boy. This is similar to Jesus dying on the cross for all of us sinners.
Throughout all the books, Aslan is portrayed as one who is omnipotent and omniscient. He is the benevolent lord who will help those who believe in him in their time of need. Just like in our world, the fictional world of Narnia also has unbelievers. There are many who have not seen him with their own eyes, and thus they do not believe.
The next few books tell of adventures in Narnia. They usually involve a king denied his rightful place on the throne. Throughout it all, Aslan will be there to help the protaganists through the darkest parts of the journey. His presence gives them courage and new strength.
I just finished reading the 7th book today. It's titled "The Last Battle". There are many parallels between that story and the Bible as well. The story starts off with an ape who dressed his donkey up in a lion's skin, and passed it off as Aslan. This happens near the end of the world. This is similar to the biblical account of the end times, which warns of false prophets. Many are deceived by this ape.
When the world ends, the animals are judged, just like how we would be judged on the last day. Some see Aslan and run away terrified, but others see him and are pleased. Those are the ones that enter through a magic doorway into another world, more beautiful than the one they left, and yet the same.
In this new paradise, the animals and people don't tire, and they are perfectly happy. This parallel is further strenghtened when it is later revealed that the children have died in a railway accident back in the "real" world.
While these books are meant for children, it was quite an interesting read. Anyway, I don't think I could have spotted so many hidden meanings if I had read them when I was younger.