I have been trying to identify a story I read in the late '70s. I think it was either written in the '30s or '40s or it was at least set in that period.
One of the main characters is a half-snake, half-human female ruler. She used some sort of magic or psychic ability to appear more appealing to humans. I remember the protagonist, who had come to like her, realizing her true form as she went to sleep, something her people did to pass the centuries.
I think the protagonist found the civilization in a valley, maybe in the Andes, by flying a plane into it. It was IIRC a technologically advanced civilization.
It has been many years, so I may have many of the details wrong, except I'm pretty sure about the snake lady and her true form appearing as she lost consciousness.