Strong Poison is just another enjoyable Wimsey novel, and the introduction of Harriet Vane. A nice touch was Sayers' decision to begin in the middle of things, at the end of Vane's first murder trial and with Wimsey already determined to marry her. Beyond that, the matter of whodunnit was rather simple, but I was led off the trail of how by a red herring. Not quite of the caliber of The Nine Tailors in atmosphere or characterization, but certainly strong.
Have His Carcase is a cut above, a truly excellent example of its genre. It is filled with unbreakable alibis, ciphers, people in disguise, a number of red herrings, and plenty of humor. I was kicking myself at the final solution, because the essential clue was one I should have seen, as I wrote a paper on it in college. The whole thing was a tremendous afternoon's fun. It doesn't advance the romance between Harriet and Peter any, but as pure detective story it is wonderful.