Even if you aren’t fully convinced by Michael Ward’s theory, it’s a very engaging read, and shows just how richly Lewis draws on classical and medieval thought and imagery in the Narnia books. It does exactly what any good work of literary criticism should do: make you go back and read the books with a renewed and deepened appreciation. It also considers C S Lewis’s other writings, including his poetry and science-fiction novels, and has fresh insights into these too.
David Field has also been enthusing about it on his blog.