Twilight Patrol

The year is 1917. After his first combat mission at the western front, Royal Flying Corps pilot Harry Tregeseal is assigned a secret assignment to a squadron in a magical otherworld, which at first seems to be a heaven-sent reprieve from the carnage at the western front.
The squadron has been sent to fight the Shroud, an enormous fleet of zeppelin-like monsters that spew torrents of poison gas. The Shroud has already destroyed the kingdom of the Unseelie Court and the threat is only growing. The lords of Faerie have invoked an ancient geas that compelled George V to support them, thus gaining themselves allies from another world.
The home of the Faerie has strange properties allowing the soldiers sent there to access abilities they'd been previously blind to. The pilots realize something strange is going on, and evidence of the intervention of the gods grows, though supposedly the old gods are all dead.
Through the peculiar goings-on, Harry and his comrades must fight against magic, weird creatures, and their own feelings to win the day.