I've given links on this page to two books that Michael Ward has written, and a DVD based on them. He has a thesis, for those who aren't familiar with it, concerning the seven Narnia books. Note that it does not relate to the films or the BBC productions - if that's your only exposure to C.S.Lewis, then you need to do some more reading before you will understand this post! The Narnia Code DVD was broadcast on network television, and became one of the most explicitly Christian things I've seen on TV. The book of the same name is accessible to children whereas Planet Narnia is quite a challenging read. However, both books effectively engage in literary analysis in an incredibly satisfying way - if your only experience is of deadly hours spent in GCSE English classes, then reading these books is an eye-opening experience. Good analysis like this doesn't kill the books, it makes them come even more alive. (That's what Bible teaching should do as well!)
His thesis is that the seven books each use one of the medieval planets to create the distinctive "atmosphere" of the book. The medieval planets are the sun and moon, Mercury, Venus, Mars, Jupiter and Saturn. The medieval view of the planets was complex - they were bound up with the characters of the gods they were named after and different aspects of the world that characterised those gods. Lewis believed that myth expressed an underlying deeper "truth", was quite fearless in co-opting this medieval framework as being representative of an underlying Christian reality. To give a brief examples of that, The Horse and His Boy is the book which relates to Mercury. It is dominated by rivers, carrying messages and twins. The Silver Chair is Luna, the moon. So there is confusion, ambiguity, a sense of "descent", madness and ... silver, the moon's metal! You can guess which planet the warlike Prince Caspian and the creative Magician's Nephew represent.
Ward's understanding of what Lewis was trying to achieve is pretty convincing, and is now apparently widely accepted. What interested me about it was the extent to which people's preferences between the Narnia books were shaped by the mood of the book, so I asked briefly which of the books people preferred and why. I suspect that the "mood" of a book does indeed have some impact on people's preferences, even if they read and enjoyed all seven books.
For what it's worth, I think that my preferred book as far as mood was concerned was The Silver Chair, even though I suspect it is one of the seven that I actually read the least. If you want to learn more about how Ward makes and defends his thesis, and how that actually works itself out, you'll have to go to the books, or at least the DVD. But once you've got an idea of how it works, go back to the books and read them again!
Feel free to comment on which of the Narnia books you liked and why. It would be interesting to see how often "mood" or "atmosphere" crops up.