Jeremy Robinson - I Am Cowboy (SecondWorld #1.5)

Please note this book was also published as Nazi Hunter:Atlantis

Milos Vesely, aka Cowboy, kills Nazis. After the events of SecondWorld-a failed attempt by modern Nazi forces to carry out a worldwide genocide and claim the planet for the Aryan nation-he now hunts down the enemy, most of whom are running for cover. But others, as Cowboy discovers beneath the sands of Tanis, Egypt, are searching for a way to strike again. Hidden beneath the "lost city" is a network of tunnels and traps protecting an ancient power that predates Egypt's mighty empires. Vesely, along with archeologist, Dr. Sarah Pasha, traverses the underground realm, heading deep into dangerous territory. But they're not alone. Sturmbannfuhrer Oskar Dirlewanger, one of the most infamous and violent Nazi leaders from World War II, and his Direlwanger Brigade, fifty of the most ruthless criminals in the Reich, are closing in, bent on retrieving the ancient power for themselves and resurrecting SecondWorld. Trapped and on the run, Vesely and his lethal aim must face off against an army, but Nazis are far from the most dangerous element beneath the hot sands. Upon finding the ancient power source, Vesely not only recognizes it, but knows where it came from: Atlantis.