I beg to differ with you, C.S. Lewis DID write The Lion, the Witch and the Wardrobe first. It was written in 1950, while The Magician's Nephew was written in 1955. See his step-son's site http://cslewis.drzeus.net/books/fiction.html - it has the information. Wikipedia (though it may not always be correct, in this case it is) has the same information http://en.wikipedia.org/wiki/The_Chronicles_Of_Narnia
What C.S. Lewis did was to wait until the sixth book he wrote to explain the origin of Narnia.
If you read the books in the order C.S. Lewis actually wrote them, you'll see that they flow very well. In fact, The Lion, the Witch and the Wardrobe makes the professor rather mysterious (which he wouldn't and couldn't be if The Magician's Nephew was meant by C.S. Lewis to be the first book read).
Here's the order in which C.S. Lewis wrote his books:
1. The Lion, the Witch and the Wardrobe (1950)
2. Prince Caspian: The Return to Narnia (1951)
3. The Voyage of the Dawn Treader (1952)
4. The Silver Chair (1953)
5. The Horse and His Boy (1954)
6. The Magician’s Nephew (1955)
7. The Last Battle (1956)
You're not meant (by the author, at least) to know the origins of Narnia before you see the strange way these children make their way into Narnia. You're also meant to know about Aslan, before you know he created Narnia with his words and song.
BTW: If The Magician's Nephew was intended to be the first of the series, C.S. Lewis would not only have written it first, he would also have never added any mystery to the professor in The Lion, the Witch and the Wardrobe. That would leave the reader saying, "Why is the writer building up a mystery here when there is none? We know the professor knows all about Narnia, why doesn't he just say so?" If you read the books in the order that C.S. Lewis wrote them, you may be pleasantly surprised.