A dapper detective tracks a high-society killer in Manhattan--from the Edgar Award-winning author who wrote the Peter Duluth Mysteries as Patrick Quentin.   Patrick Quentin, best known for the Peter Duluth puzzle mysteries, also penned outstanding detective novels from the 1930s through the 1960s under other pseudonyms, including Q. Patrick and Jonathan Stagge. Anthony Boucher wrote: "Quentin is particularly noted for the enviable polish and grace which make him one of the leading American fabricants of the murderous comedy of manners; but this surface smoothness conceals intricate and meticulous plot construction as faultless as that of Agatha Christie."   After tumbling from the Park Avenue set to penurious widowhood, the resilient Clara Van Heuten has started her own business offering counsel to aspiring writers. When it comes to advice, she's full of it. Maybe that's why she ends up with a knife in her back.   Timothy Trant, once the pride of Princeton, now one of New York's finest, uses his IQ to figure out a killer's MO. This time all the lieutenant has to work with is a stack of unpublishable manuscripts and the hoity-toity guest list of Van Heuten's last get-together--until he discovers that the widow had reason to believe she was going to be murdered . . .