John Brunner 
Wrong End of Time [EPUB ebook] 

The time is the future. The place, an America so isolated by fear that it is cut off from the rest of the world by a massive defence system. Into this armed, barricaded state comes a young Russian scientist bearing a strange and almost unbelievable story: Superior, intelligent life — on a far higher order than any on earth — has been detected near the planet Pluto. Immune themselves by virtue of their far greater intelligence, these aliens are about to destroy the planet Earth.