Heechee Rendezvous is book 3 of the classic Heechee Saga of Frederik Pohl.  While Gateway is a very different read, the second of the saga was more mainstream than its predecessor.  And HR is good solid modern Science Fiction.

Again, we join Robinette Broadhead, the even richer billionaire of the Gateway saga. But this time, right from the start, we learn of the alien race known to the humans as Heechee. The main storyline begins with a down-and-out pilot on one of the Gateway planets who decides to work his way back to Earth on a transport ship after his young wife leaves him to go exploring with the castway Han from the 2nd book. On the voyage home, he discovers evidence that the Heechee are still around and can be found, so his plans change to confront Broadhead with a plan to do so. Meanwhile, Han has found the black hole that changed Robin's life so dramatically in the first book and rescued Robin's lost love as a consequence. Both these actions alert and trigger the emergence of the Heechee into Human space, along with a warning of the Assassins who may be awakened to return and destroy all life in the universe.

Final answers in the Heechee Saga are not answered here. We are obviously led up to a conclusion in the final book Annals of the Heechee. This was a good read and Pohl's grasp of science and technology is, again, very refreshing, especially this volumes examination of Artificial Intelligences and the question of storage of human intellect.  I am really enjoying reading this series and look forward to the final book.