The Man with the Twisted Lip: Sherlock Holmes
Not a mind-bending mystery, but a wonderful story filled with devilish details. I love that Conan Doyle created a drug using/abusing main character at the height of the uptight Victorian era and in this particular story was relatively lavish with his description of an opium den and its patrons. The real essence of The Man... is its heart. The ending is quite sweet in its way.