Sentiment, Inc. by Poul Anderson

On a Cape Cod vacation, thoughtful consulting engineer Colin Fraser falls hard for bright young actress Judy Sanders, and their romance continues in New York as she fights for success on stage while he grows increasingly certain he wants to marry her. But Judy's affections begin to shift in ways that defy reason, and Fraser is soon drawn into a disturbing mystery when she reveals she has become a paid subject in a secret research project at Sentiment, Inc., where psychiatrist Dr. Robert Kennedy uses machines to measure and alter emotion. When Judy abruptly announces she is in love with wealthy industrialist Matthew Snyder, Fraser suspects manipulation rather than genuine change. Heartbroken, he turns to private detective Juan Martinez, who uncovers a pattern of beautiful young women suddenly attaching themselves to rich men after visiting Kennedy's elegant Fifth Avenue office. Fraser confronts Kennedy directly, only to be dismissed by bodyguards and left more convinced that something sinister is at work. As the investigation deepens, Kennedy defends his methods as a way of improving happiness, then reveals a chilling ambition: his machine can reshape subconscious associations, and he has used it to place himself as a trusted influence in powerful minds. The result is a tense blend of romance, psychological suspense, and speculative science about love, control, and the ethics of tampering with human feeling. This edition has been fully restored with modern typesetting, custom cover design.