A book rather like his Discarded Image in many significant respects. Fro one thing, it opens up with discussing what an epic is, on the logical grounds that's the only sane way to evaluate it.
Homer and Beowulf and the Aeneid get thrashed over. Lewis observes that only in the Aeneid do we get what we know think of as epic -- as in epic fantasy -- the large plot of enormous implications. And the style. Recited epic uses stock phrase because novelties cause the listener to ponder, and miss the next line. And later epic needs more style to aim for the sublime. More than a third of the book is filled with information and observations about the epic form as such.
And then he discusses the principle of hierarchy, and the theology of the poem. I think he goes a bit far in stating it the Christian doctrine. The ones he discusses are indeed widely held in the orthodox mainstream, but a good number are merely widely held beliefs rather than defined doctrine, even if they are compatible with it. To be sure, one he points out is that the apple is not a mystical fruit of the Tree of Knowledge of Good and Evil but merely a pledge of obedience. Since his day, more theologians have started to meditate on that tree, which is how development occurs. (That it is an apple appears to be no more than a bad pun: "apple" and "evil" both are malum in Latin.)
In the last third (or less) of the book, he settles down to the lit crit discussion of the book, and its characters, and many more interesting observations are made.