Robert Silverberg 
Across a Billion Years [EPUB ebook] 

Scattered throughout the globe of human-occupied space is evidence of a civilisation that bestrode the galaxy before humanity was born. Now, a strange device has been discovered that shows the details of that great civilisation. The details include a star map and hints that the High Ones are not extinct after all.The map beckons, and humans, being what they are, will follow. To the next great step in human destiny — or ultimate disaster.(First published 1969)