The Coming of the Terrans by Leigh Brackett

1967 ACE SF mass market paperback in good condtion. In a protective mylar case. 
 
A dying planet, dried up and hostile -- a planet of countless years and countless mysteries.

When the Terrans came, they found a world of dead sea-bottoms, lost civilizations, and secretive tribes bitterly resenting the intrusion of the Terrans on the fading glory of an ancient planet.

The Earthmen looked down upon the crumbling ruins of a brilliant culture, and laughed at the stories of invincible gods and forgotten magic lingering in the forbidden cities of Jekkara, Barrakesh, Valkis ...

But the dangers were real--and only a few renegade Earth-born adventurers who had adopted the Martian way of life could understand the planet-wide disaster that was building up.