Harry Harrison 
Down to Earth [EPUB ebook] 

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.