The World War Series oleh Joseph Altsheler

This book-collection file includes The Guns of Europe, The Hosts of the Air and The Forest of Swords. According to the original publisher: Mr. Altsheler, who was in Vienna the day war was declared on Servia in Munich when war was declared against Russia, and in England when the British forces were mobilizing, has given in these three volumes the impressions he gained at the places of action during the world crisis.... The Hosts of the Air: The pretty young sister of Phillip is seized by the enemy and carried into Austria. John resolves to get her back and his adventures make awonderfully exciting story... The Forest of Swords: The hero finds himself in Paris with Phillip Lannes, his friend, and the Germans only fifteen miles away. Finally the enemy is turned back at the Marne, a battle in which John and Phillip are actively engaged. According to Wikipedia, Joseph Alexander Altsheler (April 29, 1862 - June 5, 1919), was an American author of popular juvenile historical fiction.