"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. (This is an automatically generated summary.)
These books are free to read. Each is in the public domain in the United States; check your own country's rules before you copy or share one.