Keith Laumer

When John Bravais was sent on a secret mission to observe a war in North Africa he found out more than it was safe for him to know—even after he had secretly been surgically transformed so that he was as strong as a Bolo tank, and nearly as tough: Wolf-like aliens, invisible to the ordinary eye, were harvesting the brains of the fallen fighters! Bravais might have become the Ultimate Warrior, but still he was only one man against A Plague of Demons.