Strong Poison by Dorothy Sayers

This mystery introduces Harriet Vane, though she's by no means the most compelling supporting character in this novel--that honor goes to Miss Climpson, who runs a typing agency/investigative force. The plot is fairly straightforward as far as mysteries go, but it's endearing to see Lord Peter in love, and it does build up Lord Peter's world quite a bit. The writing is fairly elegant and erudite, as usual for Sayers. A good starting point for reading Dorothy Sayers.