Kenneth Bulmer 
Fatal Fire [EPUB ebook] 

"Mytilene, " he told them, "is a pleasure world. Or, rather, I should say worlds. You can fly from any planet to any other in the atmosphere."The whole ball, which is much larger than Jupiter, is held together by electromagnetic forces created and maintained by the machines and the robot brains on the centre planet – which spins of itself, but does not move within the gaseous envelope.