Edmond Hamilton 
Valley of Creation 

In that hidden valley, land of strangely forbidding beauty, Eric Nelson, soldier of fortune, faced a battle weirder and more savage than any he had ever fought. He was hired to fight for humanity, against beings that seemed to be both more and less than human.The weapons of the enemy included centuries-old powers of magic and superstition . . . but Nelson fought grimly, even when his mind was helplessly trapped in the body of a wolf. Then came the climactic test of his allegiance, the knowledge that more than just humanity was at stake . . . and the final mind-shattering discovery of an alien secret that lay buried in the Cavern of Creation!