His mastery of language is what made him a brilliant apologist and novel/fantasy fiction writer. He also wrote technical works on his specialty, Romance literature. If you are interested in works on medieval literature ‘The Discarded Image’ is a must read.
I have about 60 of his publications :) Quite a number 1st editions, and The Discarded Image is one I do possess.
His mastery of language is one thing yes, but without a brilliant imagination and understanding of human behavior, his writings (especially Narnia series) wouldn’t have been close to what they are.