Gaudy Night by Dorothy L. Sayers

The third detective novel featuring Harriet Vane and Sayers' recurring character Lord Peter Wimsey. Harriet returns to her Oxford college to help with a particularly nasty problem and encounters issues of eugenics, the life of the mind vs. the life of the heart, and she realizes that she must deal with her feelings for Peter because, as she comes to learn, he will never do it for her.