Here's the summary from the inner flap of Earth Ship and Starsong:

The year is 2100, and mankind has destroyed the earth's ecology. Faced with irrefutable evidence that the end is near, the technocracy that governs the remnants of the human race realizes that there is no workable alternative but to emigrate from this "bitter lesson called Earth."
But there are dangers. Man must adapt for the journey, and must perfect the power source for his Earth Ships—a drive based on the controlled creation of black holes in space. Blind to the existence of any sentient race that has not developed along technological lines, man destroys a telepathic race while refining the Black Hole Drive. Realizing its doom, this race spreads a call for revenge throughout the galaxy. The reader will follow with fascination as man—the hunted—tries time and again to found a new home, only to be destroyed and driven to the fringes of a hostile universe by increasingly virulent attacks. Will man be allowed to atone, or must he strike back to regain his place in the cosmos?