Heechee Rendezvous by Frederik Pohl

Decades after humanity first found the enigmatic technology left behind by a vanished, highly advanced race, explorers, private interests and weary veterans collide in a tense quest to locate surviving Heechee artifacts and any trace of the creators themselves; a haunted former prospector becomes central to a dangerous search that uncovers secret installations, strange intelligences and moral dilemmas, forcing people to confront grief, responsibility and the unsettling implications of contacting—and possibly awakening—an older, more powerful presence in the galaxy.