The Lion, The Witch and the Wardrobe is linked to Jupiter, Prince Caspian to Mars, the Dawn Treader to the sun and so on. For each book Ward details its links to a planet, and how the story fits in with medieval cosmology. For instance the medieval cosmos was divided into realms above and below the moon, and the 'moon' book The Silver Chair also has two realms, above and below the ground.
I had hoped for a reminder of what makes the Narnia stories so entralling but unfortuately that isn't what I found in this book. Ward writes in a very academic style and he discusses much more of C.S. Lewis's work than just Narnia, in particular the third book of the 'Planetary' trilogy That Hideous Strength which I didn't like when I read it. I think that there is much going for Ward's arguments, but the book needs more space devoted to Narnia to make them convincing If you are a fan of the works of C.S. Lewis then I think that you will like this book, but you'll need to be a fan of all of his works, not just the Narnia books.