Gateway (Heechee Saga Series #1)

"Seeded among the stars are troves of valuable artifacts left behind by the enigmatic, long-vanished alien race called the Heechee. For the right price, anyone can climb aboard one of the abandoned Heechee spaceships, cast off on an autopilot voyage to parts unknown, and take a chance on finding wealth...or facing death." Robinette Broadhead took that chance and walked away a winner. But at what cost? Despite living a millionaire's life of material luxury, he's haunted by a crippling despair - and the dark secrets buried deep in his psyche. With the help of his computerized psychiatrist, the truth about what happened "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.