Here is a list of the Heechee Saga Books in Order.
#0.5 – The Merchants of Venus (1972)
#1 – Gateway (1977)
From the blurb:
Seeded among the stars are troves of valuable artifactsleft behind by the enigmatic, long-vanished alien racecalled the Heechee. For the right price, anyone can climb aboard one of the abandoned Heechee spaceships, castoff on an autopilot voyage to parts unknown, and takea chance on finding wealth . . . or facing death.
Robinette Broadhead took that chance and walked awaya winner. But at what cost? Despite living a millionaire’s life of material luxury, he’s haunted by crippling despair—and the dark secrets buried deep in his psyche. With the help of his computerized psychiatrist, the truth about whathappened “out there” could set Broadhead free. But only after a personal journey more terrifying and, ultimately, more devastating than his last fateful trip into space.