Joseph Altsheler began writing juvenile science fiction, when there were few books in the genre. Because Altsheler felt a responsibility to his young readers, he was careful to make his books historically accurate. His simple language and interesting plots are welcome at any age. The World War Series contains The Guns of Europe, The Hosts of the Air and The Forest of Swords. The Hosts of the Air is the third and concluding book in the series. The story is primarily a love story between John Scott and Julie Lannes. All the characters from the earlier books reappear in this concluding story.