Harry Harrison 
Down to Earth 

When astronauts Gino and Dan return from mankind’s first trip around the moon, they are shocked to find Nazi Germany occupying the American southwest. Rescued by a ragtag band of resistance fighters, the bewildered spacemen learn this is not the America they left behind. Guided by the renowned physicist Albert Einstein, the astronauts embark on a daring mission to return home and prevent global domination by the Third Reich.