"Beyond the Blue Event Horizon" by Frederik Pohl

This is the second book in the Heechee series. It is less tight and organized than the previous tome, as Robinette Broadhead doesn't play as big of a part until the very end of the book. It is even more concentrated on description of technology that a super-advanced intelligent race would have, and has even less action/character development than the first volume, if that is possible. Except for some discourses of human sexuality, especially around puberty, not much of human nature is discussed, but mostly astrophysics, artificial intelligence and general futurism.

I also read this book when i was a kid, and remember very well how the Heechee solved the problem of traveling faster than light. It is different than how other imaginary FTL travel species did it, usually through traveling in 'subspace' or 'hyperspace'. The problem with FTL travel is, by Einstein's equations, the mass of a body exponentially increases with its travel speed, and if/when it reaches the speed of light, its mass is going to become infinite. With infinite mass comes infinite gravity, and a body with infinite gravity is actually an infinitely strong black hole, which will suck all of the universe into itself and destroy it. The fact that the universe still exists proves that no one has achieved travel at the speed of light in this universe.

The Heechee, however, found a way to remove mass down to 0. So a body with mass 0 could reach any speed, including and surpassing the speed of light, and its mass is still going to be zero, as any number, including infinity, when multiplied by 0 - gives 0. In the second book humans have discovered a Heechee food factory which creates edible food from comet and asteroid chunks, a Godsend for the starving (mostly) Earth. A ship is sent over, with about 3.5 years travel time, as the factory is beyond Pluto, with a crew of an old man, his two daughters, one 13 the other 39 and the husband of the older woman.

They land on the factory and discover another human boy, around 15 years old, who is driving a heechee ship to and from another Heechee mother station, huge, 1km long artifact that is called Heechee Heaven by humans. Some of the crew goes there and marvels at Heechee technology until they are caught by a tribe of Australopithecus, human ancestor species, whom the Heechee tried to make evolve quicker, but failed miserably. Also there is a sentient robot, with the memory banks of a once-living person, as the Heechee would transfer persons after their death into a computer, to continue to operate and exist.

Unfortunately the robot thinks the modern humans are pests and tries to get away, but Robin Broadhead steals a Heechee ship and gets on the Heechee heaven and together with one crew member who didn't get caught by the Australopithecus, attack and disable the robot and learn how to navigate the heechee ships and the artifact and go back to earth where he becomes the richest man in the solar system.

The book ends with a chapter about the Heechee technology and their way of thinking and how it fits in the overall cosmology of the universe, with some teasers about what is going to be revealed in the next installment of the Heechee series.