Gateway (Heechee Saga #1)

Rich or dead. Those were the choices Gateway offered. Gateway opened on all the wealth of the Universe & on reaches of unimaginable horror. The humans who rode the alien Heechee spacecraft stored on the planetoid couldn't know whether the trip would make them millionaires or corpses.
When Bob Broadhead came out to Gateway, he thought his problem was simple--wait till the mission felt right, then ship out. But watching returned prospectors scraped from the insides of their ships, falling in love, feeling his nerve dwindle--all these things changed him.
Then, years later, Robinette Broadhead, a three-mission veteran, famous & permanently rich, has to face just what happened to him & what he is...in a journey into himself as perilous & even more horrifying than the nightmare trip thru the interstellar void he finally drove himself to take!