The Miocene Arrow by Sean McMullen
In a fortieth century America of ancient kingdoms with opulent courts, hereditary engineering guilds, and rigid class distinction in warfare, a centuries-old balance of power is shattered by a few dozen Australian infiltrators. Against a rich backdrop of war, chivalry, conspiracy, and a diesel-powered arms race, a dangerous secret alliance has formed. Now the unlikely trio of an airlord, an abbess and a fugitive are joined together in a desperate race against time to stop the ultimate doomsday machine from being launched: The Miocene Arrow