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.