During his years of assigned military duty, he observed and documented the culture and traditions of several Plains Indian tribes including the Dakota Sioux. He kept detailed sketchbooks in pencil and watercolor that captured the landscape and peoples of the frontier.

In 1847, Eastman won the Congressional commission to illustrate Information Regarding the History Condition and Prospects of the Indian Tribes, with text by Henry Rowe Schoolcraft. The engravings, (mostly black and white but some printed in sepia) depicted artifacts, pictographs, and genre scenes.