Robert Silverberg 
Nightwings [EPUB ebook] 

Old Earth has reached its Third Cycle, a tired planet basking in the faded glories of a lost civilisation. Long ago it had been great — but the pride and greed of its rulers had brought about a terrible downfall. And now Earth was threatened. Far out in space an alien race waited. Once they had been the victims of a crime perpetrated by the human race — now they were ready to return as conquerors. As a Watcher, Tomis had spent his life searching the skies for signs of the impending invasion. And when it finally came, it was to disrupt not only his world but his whole life in a way he had never dreamed was possible.(First published 1969)