I don't think that most Holmes stories should even be read as we do modern mystery stories. They are adventure stories, and Conan Doyle is more like an intellectual Raymond Chandler than a precursor to Agatha Christie (although I suppose, in fact, that he is both). It's simply impossible to solve most of the early ones (including A Study in Scarlet), although the later stories (which postdate Christie's first stories) were more honest mysteries. (At one point he even has Watson apologise for having been unfair in the past.)