I'm not sure how to describe this book but I do know how I feel about it. For many years it was my favourite (don't tell the children) but it lacks the "save the world" story line so common in fantasy books. The story is driven by the engaging characters and what happens to them in the passing lands and seas. Sounds rather pedestrian, yet the book is as gripping as any other in the series.
One of the characteristics of Lewis' writing is his vivid imagery, helped along a little by Pauline Baynes' illustrations. There are scenes that remain, like an afterimage, for years after the plot has faded. I can see Lucy walking down the corridor or Prince Caspian having the Mayor discomfited or the weeping dragon.
Perhaps it is the prospect of being drawn into a painting, escaping the dreary mundane life of the drawing room that sparks the magic of this third volume. Enjoy.