Sequel to McMullen's impressive far-future yarn, Souls in the Great Machine (1999). By the 40th century, civilization is struggling to be reborn into a world devastated by a strange, ancient war that has left its imprint deep on the present. The Call radiates from somewhere under the distant ocean, sweeping the land every few days or hours; any mammal above the size of a cat is seized and compelled to trek mindlessly westward until it dies or topples into the ocean. Orbital forts known as Sentinels are still active, and instantly blast anything that moves if it's more than 30-feet long. And a mysterious damping field prevents electrical devices from operating. In America, enclaves of civilization have developed in Callhavens—pockets of land in the mountains where the Call comes only every few days, so ways to counter it have been developed. And the pilots of gunwings, flying machines powered by compression engines, aren't subject to the Call if they remain more than 50 feet up. These flyers form the Callhavens' aristocracy. The Callhavens feud and plot and scheme against one another, activities spurred by the unsuspected presence of spies from halfway around the world.
Once again, McMullen's machines are miracles of low-tech inventiveness; but the majestic Calculor belongs to the previous book, and the present characters, homelands, and motives remain indistinguishable. Overlong, overinvolved, and disappointing.