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.

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