Considered one of the great books of the genre, I direly wanted to re-read it after I read the sequel Beyond the Blue Event Horizon recently. That's an unimpressive book: this, on the other hand, has re-affirmed itself as one of the best SF novels ever written.
Our main character is Robinette Broadhead, living the high life on an over-populated Earth. He's seeing a computerized psychologist he calls Sigfrid, who over time slowly pushes him toward a deeply traumatic moment in his life. The novel alternates between his sessions with Sigfrid (in which he mostly avoids talking about his past) and flashbacks to his time on Gateway, which is an alien artifact circling the Sun left half a million years ago by an alien race known as the Heechee.
Gateway is littered with Heechee ships. The problem is, humans don't know how to fly them. You can make them go, but where you end up is a total crapshoot. And whether you come back is equally a crapshoot. Not too surprisingly, Robin is terrified to go. And he meets a woman on Gateway, Klara, with similar issues. But eventually they go out together - a long trip in a small tin can that produces nothing of value ... a trip that deepens their relationship in ways both good and bad.
All of this and much more is slowly and painfully revealed through the course of the book - interspersed with mission reports, personal ads, letters home, and various other Gateway documents that really add flavour to the place. The ending is seriously shocking, and still a hard thing to read even knowing the outcome from my previous (long ago) reading of the book.