Three crushed roses intiating violence a cypher message and the picture of a beautiful woman known to be an international spy--that's what Harleston gentleman and member of the Secret Service found in a cab deserted by all but the horse sleeping between the shafts at 1 a.m. on Massachusetts Avenue. And before dawn he was set upon in his own apartment. Mr. Scott knows how to tell a charming love story and arouse intense excitement at the same time.