Allegory, in some sense, belongs not to medieval man but to man, or even to mind, in general. It is of the very nature of thought and language to represent what is immaterial in picturable terms. What is good or happy has always been high like the heavens and bright like the sun. Evil and misery were deep and dark from the first. Pain is black in Homer, and goodness is a middle point for Alfred no less than for Aristotle. To ask how these married pairs of sensibles and insensibles first came together would be great folly; the real question is how they ever came apart, and to answer that question is beyond the province of the mere historian.
C. S. Lewis The Allegory of Love: A Study in Medieval Tradition, 44.
Oh! But you should search for a used copy on Bookfinder.com (where I got mine) and/or Addall.com. There are many inexpensive copies! I was lucky to find an Oxford hardback for under $30, through patience.