Basil II.

A mysterious test appears on every screen on Earth: "Application Exam for Starship Commander." When novelist Joseph Doherty clicks out of curiosity, he is abducted into orbit and chosen by an alien AI as humanity's reluctant leader. Now, with Earth on the brink of annihilation, Joseph must rebuild civilization from the shadows-balancing stolen alien technology, fragile alliances, and a moral credit system that trades virtue for knowledge.
But as the Homeworld Civilization rises and a fleet gathers above Mars, Joseph begins to suspect that the real exam has only just begun. and that his "recruiters" never left the solar system