The story is the myth of Cupid and Psyche, told from the POV of Psyche's eldest sister. In the traditional myth, Psyche is so beautiful that Aphrodite is jealous and orders her sacrificed. She is rescued by Cupid, who takes her to his home but prohibits her from ever seeing him. Her sisters go to visit and are so jealous they trick her into looking at Cupid. When she does she is exiled and wanders the Earth for eternity.
In this version the eldest sister is not jealous, but rather she loves Psyche. When she visits Psyche on the mountain in Cupid's home, she cannot see its wonders. She believes Psyche is captive of a monster or a delusion, and coerces Psyche into looking at Cupid in the hopes she'll come to her senses and return home with the sister. Psyche does so, and is sent into exile.
Lewis' take on the myth is pregnant with messages about faith and doubt. It's written as an indictment of the gods' demands that we believe things we cannot see or verify.
I read it first back in 1999 on my father's recommendation. I remember really liking it a lot. I liked it enough that it was the first book I ever gave to my then-girlfriend (now wife). She also liked it a lot at the time, but doesn't really remember it now.
Honestly, it's very good but I can't really see whatever it was that I saw in it the first time. I know I'm in a very different place, spiritually, than I was then. That might have something to do with it. Back then I was learning a lot about varieties of spirituality. Today, I'm much more settled into my own space, and I don't think it's very similar to C.S. Lewis' questions and agonies in this book. (Contrast with The Screwtape Letters which speaks to me very precisely).
Music and silence–how I detest them both!….[Hell] has been occupied by Noise–Noise, the grand dynamism, the audible expression of all that is exultant, ruthless, and virile–Noise which alone defends us from silly qualms, despairing scruples and impossible desires. We will make the whole universe a noise in the end….The melodies and silences of Heaven will be shouted down in the end. (C.S. Lewis, The Screwtape Letters, 119-120)