THE JUDAS TREE was a ""dark horse"" success last Spring. THE PHANTOM EMPEROR is just as good a yarn, and an even more unusual historical background. Swanson seems to have a genius for ferreting out extraordinary episodes in our own history, and giving them reality. This time ""the phantom emperor"" is a Frenchman, an officer under Napoleon, who thinks himself destined to be an emperor of what is now our southwest and California. By force of his own belief and his dominant personality and charm -- and , in no small measure, through the personality of his daughter, a girl raised as a son and believed by many of her father's followers, to be a youth -- this amazing man collected equipment and ammunition and supplies, and gathered a band of men, half breeds and Frenchmen, and set forth by boat, from Buffalo, westward. Deprived of their ship and a large part of their supplies at Fond du Lac, they struggled through a terrific winter in the wilds of Minnesota, and the farce was brought to an end with his death during a battle between the Dakotahs and the Ojibways. Against this factual background, incredible as it seems, is told a thrilling adventure story, and a glamorous romance. Now and again, the story seems to be swamped in the intricacy of its pattern, but it is good reading throughout. The publishers are planning to back it up with advertising in metropolitan newspapers. The history of THE JUDAS TREE should make it easy selling.