Heechee Rendezvous

Advanced Heechee technology had enabled Robinette Broadhead to live after death as a machine-stored personality, enjoying his life by flitting along the wires from party to party with a host of other machine-people. But suddenly his decadent existence ends when an all powerful alien race intent on the utter destruction of all intelligent life reappears after eons of silence, and threatens the lives of all heechee and humans. Even Robin, virtually immortal and with unlimited access to millennia of accumulated data, cannot discover how to stop these aliens. It began to seem that only a face to face meeting could determine the future of the entire universe....