The Moonflower — Phyllis A. Whitney

When Jerome Talbot's brilliant scientific career led him to Japan, his wife, Marcia, knew it meant a long separation for them. But she confidently awaited word to join him and when, instead, she received his letter asking for a divorce, the news came as an overwhelming blow. In shock and dismay, she flew at once to her husband in Kyoto, hoping to wrest from him some reason for his determination to end their marriage.
When she arrived, she discovered Jerome living on terms of extraordinary intimacy with the Japanese family in the house next door. He forbade Marcia to see them or talk to them, and yet she could not help but see that he was in some mysterious way at their command. For the rest, she met a blank wall of indifference in him, broken occasionally by a bitter wish to humiliate her.
Though fear and frustration invaded her heart, Marcia kept to her purpose. If she couldn't win back Jerome's love she could, for the sake of their happy years together, try to help him with a problem -which she did not understand, but which she felt threatened his sanity.