Lost Worlds of Cronus — Colin Kapp

Mercury Shell, Venus Shell, Earth, Mars, Asteroid, Jupiter, Saturn. Each shell concentric, studded with artificial planets, each planet embedded in its shell, spinning like a ball-bearing. The whole Zeus-created in the service of Man but now beyond his control.
Now mathematics and space physics, converging, suggested another shell, its existence hidden from Man. A shell of utter darkness, cold and silence where only extreme mutants could survive.
To find that shell, the three were journeying again: Maq Ancor, Master Assassin, Magician Cherry and Sine Anura, Mistress of the Erotic. Together, daring the all-seeing, all-sensing hostility of Zeus.