Edmond Hamilton 
Star Kings 

Flung across space and time by the sorcery of super-science, John Gordon exchanges bodies with Zarth Arn, Prince of the Mid-Galactic Empire 2000 centuries in the future! Suddenly John is trust into a last-ditch battle between the democratic Empire World and the tyranny of the Black Cloud regime. Only one weapon – the terrifying Disruptor – can win the struggle for the Empire Forces. But it is so powerful that unless John uses it correctly it could destroy not only the enemy but the cosmos. Could his 20th Century mind cope with the technology of 200, 000 years from now?