From back cover Del Ray paperback May 1988:

HORIZON'S GATEWAY

Advanced Heechee technology had enabled Robinette Broadhead to live after death as a machine-stored personality. He passed the time flitting along the wires from party to party with a host of other machine-people.

But suddenly his decadent existence was interrupted by disaster -- a powerful alien race, intent on the utter destruction of all intelligent life, had reappeared after eons of silence, and the lives of all Heechee and humans were at stake.

Even Robin, virtually immortal and with unlimited access to millennia of accumulated data, could not discover what these aliens were -- or how to stop them. Yet it looked as if he was the only one able to deal with the enemy face to face -- a meeting that would determine the future of the entire universe.