Michael Ward wrote the more substantial
Planet Narnia: The Seven Heavens in the Imagination of C. S. Lewis to set forth the concept that the Narnia books each echo the characters of the seven mediaeval planets. It is an excellent book: however, it is a challenging read - unsurprising, as it is fundamentally a PhD thesis. He was invited to write a book defending this thesis at a more accessible level, and "The Narnia Code" is the outcome.
More than being a stripped down version of the other book, there is new material here - there's value in having both books. Ward has continued to explore the thesis, and the evidence in support of it is very strong, on the level of motive, opportunity and means.
A book about books always runs the risk of taking away the magic. In my opinion, as with
Arthur Ransome and Captain Flint's Trunk, this book doesn't do this at all - it adds to the magic, once you can see how cleverly Lewis has used imagery to create the work. I would strongly recommend this book for anybody who has enjoyed the Narnia books enough to read more than one of them.