Planet Narnia, by Michael Ward, is one of my recent favourites. I recommend it to anyone who has enjoyed C. S. Lewis's Chronicles of Narnia.
I came across Planet Narnia in the library at Huron University College while searching for books to read on another book by C. S. Lewis, Till We Have Faces. (If I may digress, Till We Have Faces, in my view, is Lewis's best novel; indeed, I would go so far to say that, excepting some of his better academic works, it is his masterpiece). I read it voraciously, and so thoroughly enjoyed it that I wrote a review of it which I posted on my defunct LiveJournal, and even went so far as to post a portion of that review on a site dedicated to reader reviews of Planet Narnia; you can see my humble contribution here.
In my view, Planet Narnia is to the Chronicles of Narnia as Tolkien: A Cultural Phenomenon is to The Lord of the Rings; that is, it is a work of literary criticism the insight of which is so brilliant it outshines other works. Put another way, I view Planet Narnia as a sun with respect to the Chronicles of Narnia, brightly illuminating every place. Other works of literary criticism on the Chronicles are like the stars, providing a dim and feeble light but often pretty to look at in their own right, or else like the moon, capable of brightening some aspect of the Chronicles but not as strongly as the sun can.
But what is it about Planet Narnia that makes it such an insightful and penetrating work? To discover that is the purpose of this marginal commentary, by peering closely at Planet Narnia and seeing by what means Dr Ward appears to so accurately delve into the Chronicles of Narnia.