A mysterious message signals an international threat in this World War II thriller by an Edgar Award-winning author.   A French refugee is washed up on the English coast. Feverish, delirious, he babbles incoherently to the men who find him. A single phrase, repeated: "Loftus, spell it backwards."   This discovery sparks an explosive reaction throughout Department Z, whose agents--including Bill Loftus--are fighting a desperate battle to uncover a Nazi scheme that threatens the very heart of British defense.   After startling revelations of a conspiracy theory, a kidnapping plot, and a dangerous weapon, Department Z just may find themselves in over their heads this time . . .