My father mentioned a book he read back in high school about two people (a girl and a boy) that were transported into a fantasy world. Either they were two teenagers/students, or two scientists that were lost, he's not sure. He mentioned that the longer the two stayed in this particular world, the more they began to change into two species of a fantasy race. The girl/woman began to turn into a pixie, and the boy/man began to turn into a goblin type creature.
The main thing that he says got a kick outta him was that the pixie dust, aka the thing that was always seen as magical in other stories, was pixie poop in this universe. A clever idea and joke in my opinion.
My father was born in the early 60's, so I imagine that if the book was new at the time, it would probably have been published in the late 70's/early 80's.
I know this is a shot in the dark, but does anybody happen to know of such a story?