Robert Silverberg 
Letters from Atlantis 

It is the 21st century, and a man can journey through time in the form of disembodied consciousness. Now Roy Colton is about to leap 20, 000 years into the past. His mission: to observe the fabled city-island of Atlantis through the mind of its royal heir, Prince Ram. But Roy is soon worried by Ram’s dark dreams. Do they presage the island’s legendary destruction? And how can Atlantis be so technologically brilliant during a barbaric ice age? Boldly, Roy seeks the truth, yet nothing can prepare him for the strange secret of Atlantis’s past…and future.