I Will Fear No Evil(1970)

Robert A. Heinlein

Johann Sebastian Bach Smith was immensely rich, and very old. Though his mind was still keen, his body was worn out. His solution was to have surgeons transplant his brain into a new body. The operation was a great success, but the patient was no longer Johann Sebastian Bach Smith. He was now fused with the very vocal personality of his gorgeous, recently deceased secretary, Eunice, with mind-blowing results, for Eunice hasn't completely vacated her body...

Together they must learn to share control of her body. Mind versus body, Masculine desires versus feminine appeal. And soul versus soul. And when a third soul joins them the situation becomes even more fantastic. . .