Todd Downing? Perhaps other aficionados of classic detective stories will recognize his name, but until recently I had never heard of him. Not even Jacques Barzun and Wendell Taylor in their “Catalogue of Crime” carry an entry on any of his books. And yet — from the evidence of Downing’s mystery criticism, as well as his fiction — he seems to have been just their kind of author: Not just well read and literate, but also devoted to the fair-play puzzle in which the reader can match wits with the detective over who done it and how.