Beyond the Blue Event Horizon

Years after his first trip through a Heechee tunnel made him rich, Robin Broadhead funds an expedition to a newly found Heechee 'food factory' drifting in the Oort cloud, partly to feed an overcrowded, starving Earth, and partly because he can't stop thinking about Klara, the woman he left behind in a black hole years before. The crew finds the factory already occupied: a young man named Wan, raised alone on a distant Heechee station, who has been visiting the factory for years using alien technology no human understands. His visits, it turns out, aren't as harmless as they seem. The second novel in Frederik Pohl's Heechee saga, following Gateway.