The Judge and His Hangman (1950)
by Friedrich Dürrenmatt
Friedrich Dürrenmatt was a Swiss author, best known for his "Brechtian drama" but he dabbled in detective fiction. Barely one hundred pages in length, The Judge and His Hangman fuses the conventions of detective fiction with a heavily existentialist theme. It's impossible to delve deeply into the plot without giving up what makes this story worth seeking out in the first place, but by all appearances the first 20 or so pages read like a conventional detective mystery in a Swiss town. After the traditional opening you get plunged into a convoluted battle of wills between a dying police detective and a kind of Nietzchean super villain.
The Judge and His Hangman can also be read as a precursor of the "Nordic Detective" novel tradition, most notably The Girl with the Dragon Tattoo trilogy.