To tell the truth, I read the book years ago and only remember the basic plot, thinking it was OK. I was mostly aggravated that I'd bought it without realizing it was Book #3 in a trilogy. You don't actually have to read books 1 and 2 first, because you can get the gist without, but I'm sure they would help get the character nuances.
From back cover: A PROBLEM IN RELATIVITY
ONE: Eons ago, a gentle race of giant aliens fled the planet Minerva, leaving the ancestors of Man to fend for themselves.
TWO: 50 thousand years ago, Minerva Exploded, hurling its moon into an orbit around the Earth.
THREE: In the 21st century, scientists Victor Hunt and Chris Danchekker, doing research on Ganymede, attract a small band of friendly aliens lost in time, who begin to reveal something of the origin of Mankind.
Finally, Man thought he comprehended his place in the Universe... until he learned of the Watchers in the stars.
From back cover: A PROBLEM IN RELATIVITY
ONE: Eons ago, a gentle race of giant aliens fled the planet Minerva, leaving the ancestors of Man to fend for themselves.
TWO: 50 thousand years ago, Minerva Exploded, hurling its moon into an orbit around the Earth.
THREE: In the 21st century, scientists Victor Hunt and Chris Danchekker, doing research on Ganymede, attract a small band of friendly aliens lost in time, who begin to reveal something of the origin of Mankind.
Finally, Man thought he comprehended his place in the Universe... until he learned of the Watchers in the stars.