Gaudy Night by Dorothy Sayers is a classic mystery novel. There is a murder, but there's also an understated and ongoing courtship, a tangle of relationships and choices, and the decision of its heroine about the path she'll take in future. It will make you wish you were an Oxford scholar. The writing is superb.
"Harriet flung her window open and leaned out. There was a sweet smell of approaching rain. Another flash and crash; a swift gust of wind; and then the swish and rush of failing water, the gurgle of overflowing gutters, and peace."
A perfect ending to a tense and building chapter. One of my favourite snatches of writing.