Star Trek III: The Search for Spock (Star Trek: The Original Series (numbered novels), #17)

As the crew grieves for Mr. Spock, the awesome Genesis Device, now controlled by the Federation, has transformed an inert nebula into a new planet teeming with life. But Genesis can also destroy existing worlds.

The creators of the Device want it given freely to the Galaxy. But Starfleet Command fears that it will become a force for evil. And the enemies of the Federation will not rest until they seize it – as their most powerful weapon in the battle to conquer the Galaxy.