Gateway by Frederik Pohl
Wealth . . . or death. Those were the choices Gateway offered. Humans had discovered this artificial spaceport, full of working interstellar ships left behind by the mysterious, vanished Heechee. Their destinations are preprogrammed. They are easy to operate, but impossible to control. Some came back with discoveries which made their intrepid pilots rich; others returned with their remains barely identifiable. It was the ultimate game of Russian roulette, but in this resource-starved future there was no shortage of desperate volunteers.