Frederik Pohl - Black Star Rising

It's the late twenty-first century. The USA and the USSR have destroyed each other in a catastrophic nuclear exchange, and China now rules the Americas. Then a mysterious alien spacecraft approaches Earth and demands to speak to the President of the USA. There's been no such person for decades; but the aliens won't take no for an answer, and vaporize a Pacific island to prove it. A puppet President must be found to go into space and placate the aliens. Pettyman Castor, a humble rice-cultivator from the Heavenly Grain Collective in Mississippi, is chosen to play the part, and the peasant turned President is soon on his way to an alien galaxy - and the bizarre revelation that awaits him there will turn his world inside-out...