Vanguard From Alpha — Brian W. Aldiss

The spy team from Earth knew they were looking for trouble when they landed on Luna Area-101 dangerous Rosk territory. But the fearless trio got more then they bargained for and the hands of these hostile guests of Earth. Tyne and Murray escaped with their lives. The third man was dead, and Tyne suspected that Murray had murdered him in cold blood. Ready to confront him with his charge, Tyne discovered that Murray had disappeared somewhere in the banned area. But when he followed him, he discovered something vastly more dangerous then Murray's guilt or innocence---the Rosks threatened imminent invasion of Earth. And only Tynenow held the secret that could deflect their hordes of death. Yet he trusted no one with it---not even himself.