Earthlight by Arthur C Clarke

Two hundred years after humans first touched down on the surface of the Moon, there are permanent settlements there—as well as on Venus and Mars. The inhabitants of these colonies have formed their own political alliance: the Federation.
On the Moon, a government agent from Earth is hunting a suspected spy at a prominent observatory. He is caught up in the larger political struggle between Earth's government and that of the Federation, and ultimately must struggle for his life—in the beautiful and barren landscape of the Moon under Earth's light.