As Jack points out in his introduction, this booklet was written more as an answer to The Marriage of Heaven and Hell, while drawing upon some of his favorite classical literature and its characters (namely the Divine Comedy).
This book reflects Lewis's disdain for the premise of dualism, that is, that good and evil are co-real, co-existant, and equal forces within the universe. Rather, he believes that the nature of "good" (and its reality and existence) is fundamentally different from that of "evil". According to Lewis, evil is by nature a corrupt derivation, a sham, an ill-conceived, poorly-executed, unoriginal attempt to mimic the good in part while rejecting the whole; good is the fundamental, the one true axiom, being in its nature the expression of the nature of God, the creator, the prime mover. Thus the heavenly kingdom's incredible density - the visitors from the netherworld are themselves but shadows, having lived for so long amidst nothing but shadows, and as such they have great trouble abiding or even comprehending the substance of which all they know are poor reflections.
"Nothing, not even the noblest, can go on as it it now is. Nothing, not even what is lowest and most bestial, will not be raised again if it submits to death. It is sown a natural body, it is raised a spiritual body. Flesh and blood cannot come to the Mountains. Not because they are too rank, but because they are too weak. What is a Lizard compared with a stallion? Lust is a poor, weak, whimpering whispering thing compared with the richness and energy of desire which will arise when lust has been killed." The Great Divorce, p.104-105