I'm sorry, I'm still obsessed with how the mystery structure is woven into Harriet and Lord Peter's romance.
Wimsey had his whole "love at first sight" thing, but he also got to spend an entire book investigating a mystery surrounding Harriet. While she was stuck in prison, he got to talk to people who had known her in her former life, got to meet her friends and learn about her background and discover all sorts of details about who she is as a person.
Harriet never got to do that for Peter. She just knew him as an aristocrat, as a detective, and as someone who wanted to marry her. She interacted with people who knew him as one of those three things, and she got to know him in the context of a relationship where she never got to see much beyond the surface.
In Oxford, Harriet gets to dig into Peter's past. He stops asking her to marry him, which finally gives her the chance to ask questions about him. She meets people who knew him before he knew her, who know him in lots of different roles. She gets to see what he's like as an uncle, how he deals with disciplining an irresponsible son of the family. She learns about his work with the Foreign Office. She meets an old college friend who knew him as the silly aristocrat you show off to your friends. She meets someone who knew him as a commanding officer. She even gets to watch him sleep--which is explicitly, with multiple allusions, compared to death--then rummages through his pockets gathering evidence that gives clues about his personality.
She learns what made him fall in love with her. She learns that other people can fall in love with him. All this is evidence that lets her piece together a picture of the history that led him to this moment, and helps her figure out how she feels about him.
He got to investigate her, and her investigation of him puts them on equal-enough footing to marry. The head's not getting in the way of the heart--it opens up the heart, because for them, knowledge leads to love.