A Man and His Mother-in-Law — Roy Vickers

Highly entertaining "inverted" mystery stories (the reader knows who committed the crime, but the interest lies in how the detective solves the case) that appeared as a series in a British magazine in the 1930's. Vickers posits a (fictional) branch of Scotland Yard, the "Department of Dead Ends," a repository for the unsolved cases of the yard. It's up to the brilliant detectives of this branch to solve these seemingly impenetrable mysteries. Vicker's characters are full fleshed and his plots ingenious.