Marion Zimmer Bradley
The Planet Savers
"The Planet Savers" by Marion Zimmer Bradley is a science fantasy novel first published in 1958. When a deadly fever threatens to devastate the planet Darkover, a psychiatrist takes a desperate measure: unleashing his colleague's hidden personality through hypnosis. The gregarious Jason must lead a dangerous mountain expedition to find a cure, but he shares a body with the cold, clinical Jay—and their personalities keep switching at the worst possible times. As the mission unfolds, Jason falls in love, complicating an already precarious situation.