There is an interesting and very favourable review by Tom Wright, Bishop of Durham, in a recent TLS of Planet Narnia, a new book on C.S.Lewis, and in particular the symbolism of the planets embedded in the Narnia series. It argues that themes that are explicit in The Discarded Image and That Hideous Strength can be used as a method of understanding the seven books of the Narnia series.
The author, Michael Ward, has put together a website dedicated to the book. It is available online (to subscribers) at Oxford Scholarship Online.