That's the question Juliet asked. It's also been a matter of discussion in book club discussions about 'The Kingdom of the Air'. What is the significance of the title? The obvious reference is to the contested skies during the Battle of Britain - a test of air superiority where the entire course of human history hung in the balance. But there is a lesser known reference. The phrase -'The Kingdom of the Air' - is taken from the Bible. In Ephesians 2 we read:
As for you, you were dead in your transgressions and sins, in which you used to live when you followed the ways of this world and of the ruler of the kingdom of the air...
The entire passage is about death and rebirth. By grace, someone who was 'dead', and following an evil ruler, can be made new.
There is a theory that there are really only seven types of plot in literature. One of those is the 'Death and Rebirth' narrative. The Kingdom of the Air follows this traditional dramatic form. Josef is 'dead', following an evil ruler (Hitler) and doomed. It is only by an act of grace (Giselle's determination to help him defect) that he is spared, and made alive again... hence the title of the novel; an allusion to the greatest story of all.