At one time Isaac Asimov wrote novels that were not connected to a series, and this is one of them, though later in life he tied it into his Foundation/Robots universe. This is a good enough novel and Asimov is a good writer, but this is not the best thing he ever wrote, even though it was on the short-list for a Hugo. Most of the novel is about a group of people in an organization called Eternity that have time travel technology. They have taken it upon themselves to preserve the safety of their timelines by making "reality changes". But the protagonist, Andrew Harlan, is eventually placed in a position where he needs to decide if this is such a good idea. He has fallen in love with a woman from the "hidden centuries" who tells him that in every time line where Eternity has smoothed things, the human race dies out, but when there is no Eternity, the human race dominates the galaxy.