Jag tror att Psychons herrar är boken du letar efter.
The novel is set 15 years after the devastation of earth. A band of holdout soldiers remain steadfast to their military/defensive duty, and are based in a deserted junior college in the shadow of a strange, towering, alien City of Force. Wherever the aliens have established themselves, they have raised up great walled cities comprising huge buildings shaped like cylinders, pyramids, cubes, rhombohedrons, semi-transparent prisms, and balls. Electric equipment all over the earth has been shut down by the aliens, and all of human civilization has been driven back to a coal and steam-based technology. The Cities of Force are the home of the “Spheres”. They are large floating aliens that generally ignore humans except when they decide to “select” someone. Then they hover and float through the air, crackling with static electricity and hunting the selected person apparently for sport. Spheres can travel right through walls and hillsides in their pursuit of the selected people. When they finally run down the prey, they throw lightning bolts to kill the human. The Spheres also get active around the time of Horror Day, an annual event when a grid of energy forms over the earth and somehow inflicts incredible physical and mental torture on the earthlings, which takes months for the populace to recover from.