Four Days' War — S. Fowler Wright

The Nazis attack the West--in 1938! First their Blitzkrieg attacks and subdues Czechslovakia. Now it's England's turn, as wave after wave of German bombers devastate London and the other big cities. British spy Richard Steele has evidence of an appalling Nazi weapon, a gas that freezes everything it touches. But Steele's stuck in Germany--can he return home in time to warn Britain? The second book in a gripping trilogy.