vintage Ernest Ritter Grand Canyon midget horses photo postcards/booklet

Old 1930s or 1940s vintage set of photo post cards and accompanying booklet by artist and explorer Ernest Ritter. The four postcards have never been used. According to the booklet these are the world's smallest horses, some no bigger then dogs. They were found trapped in part of the Grand Canyon and are the descendants of Indian ponies trapped there when the trail collapsed.