The Givers by Bill Larsen

After a peaceful alien population cohabited the Earth for years with humans, they suddenly turn violent, eradicating most of the population, starting a great war between humans and these aliens, who they call The Givers, and vacating the planet.

Twenty years after this war, Earth has kept peaceful communications with The Givers. That is, until now. Kort Oliver has been selected by the head of Earth’s new government to be a member of an elect team sent to investigate the recent silence on the Givers’ home planet, where no human has ever been.