Now Wait for Last Year, Paperback/Philip K. Dick

Earth is trapped in the crossfire of an unwinnable war between two alien civilizations. Its leader is perpetually on the verge of death. And on top of it all, a new drug has just entered circulation--a drug that haphazardly sends its users traveling through time. In an attempt to escape his doomed marriage, Dr. Eric Sweetscent becomes caught up in all of it. But he has questions: is Earth on the right side of the war? Is he supposed to heal Earth's leader or keep him sick? And can he change the harrowing future that the drug has shown him?