Gaudy Night
The best novel of the best mystery writer of her era, Gaudy Night is a wonderfully compelling story of an outbreak of vandalism at an Oxford women's college in the 1930's.
Hammett, of course, wrote in the same period. He's a better writer; his prose is more innovative, his characters fresher, his world more real. But Hammett looks forward, sometimes far forward; the original plan for The Maltese Falcon was to present the whole affair as stream-of-conscious. Sayers looks back to Doyle and Galsworthy; indeed, when she populates the display case of the Shrewsbury College Library with a First Quarto and a signed first edition of Galsworthy's The Man of Property, we realize with a start how much the literary landscape has changed.