Till We Have Faces, turned out to be his last. But, while quite different than his science fiction writing, was just as masterful, and I give it my highest recommendation. This work amounts to a retelling of the mythological story of Psyche and Cupid, but with Lewis's signature touch.
The story takes place in the kingdom of Glome, located on the outskirts of the Hellenistic world. When King Trom loses his wife, he seeks to quickly remarry so that his maid can produce a male heir for him. Thus far his offspring consist of two daughters, Orual and Redival. The king's new wife is quickly with child, but dies giving birth to another daughter, Psyche. The child is unmatched in beauty. The ugly Orual loves Psyche as a mother and devotedly takes to her upbringing. Ultimately, the townspeople of Glome come to worship Psyche for her beauty and healing touch, instead of worshipping the local goddess of nature, Ungit.
Following a devastating plague, drought, and famine, the high priest of Ungit tells the king that relief will come only if Psyche is sacrificed to Ungit's son, the Shadowbrute. The king complies to save his crown and gives up his daughter for sacrifice. Later, Orual goes to bury Psyche's bones and finds her alive and hale. Psyche invites Orual to her palace, which is invisible to Orual, and speaks of a husband that she is not allowed to see. Orual convinces Psyche to take a lamp in at night and look. Psyche is discovered and banished from her lands. Orual becomes the queen of Glome upon the king's death and lives the rest of her life in dread and misery for the loss of her sister, for her virginity and lot, and for the unfairness of the gods. Just before her death, she is allowed by the gods a glimpse of perspective. How she has loved, but has loved selfishly and with a misguided heart. Ultimately, everyone is dehumanized by their lack of love or their egregious mis-steps in what they think is love, so that even though people can still see their faces, they are just mere illusions.