The Dorothy Sayers books are wonderful, and I especially like the Lord Peter / Harriet Vane ones. I would recommend that you start with Strong Poison. The next ones in line after that are Have His Carcase, Gaudy Night (my favorite), and Busman's Honeymoon. (There are a couple more written by Jill Paton Walsh in the last few years that are OK, but not nearly as good as Sayers'.) One that does not have Harriet in it but that I particularly like is Murder Must Advertise. But really, you can't go wrong with any of her books.