In case you are referring not to non-fiction, but specifically historical works written by Lewis, there are two major ones.

First is The Allegory of Love: A Study in Medieval Tradition, his major scholarly work and still sufficiently influential as to still be cited occasionally in a constructive manner.

Second is The Discarded Image: An Introduction to Medieval and Renaissance Literature, which is more of an introductory work (at least, not having read this work, I infer this from the fact that I haven't seen it cited alongside authors like Gurevich in the relevant literature).