Brunner, John. Double, double. New York: Ballantine Books, 1969.

The central idea and some of the plot details are lifted from John Campbell's classic horror story “Who goes there?” with a civilian setting replacing Campbell's isolated military base. Some of the characters are stereotypes of late-sixties youth, affectionately rendered but now painfully dated. (To give you an idea: There is a rock band called “Bruno and the Hermetic Tradition.”)