Sketches from the Missouri river Expedition, 1932-1934, for which he was hired by a German explorer to document the native peoples they encountered. On his return to Europe, the sketches were reproduced as color aquatints and made into a book. I'll gather some of the colored editions, as well. This selection shares the range of information he compiled and shows how thorough his documentation was. And they are beautiful.
Indian Utensils and Arms
Indian Utensils and Arms
Drawing of Native North American Bows, Quiver and Arrows
Painting of Bison Head
Map of Fort Clark and the Mandan Village of Mih-Tutta-Hang-Kush

Drawing of Topographical Map Showing Routes of Exploration, Mandan Villages and Missouri River
Watercolor Painting of North Dakota Landscape
Watercolor Painting of Standstone Mesas
Painting of Indian Warrior
Drawing of Elderly Plains IndianDrawing of Omaha Indians
All images from corbis