Eastman went on to become one of the best-known Indians of his time, receiving a Bachelor of Science degree from Dartmouth in 1887 and a medical degree from Boston University three years later. From his first appointment as a physician at Pine Ridge Agency, where he witnessed the events that culminated in the Wounded Knee massacre, he sought to bring understanding between Native and non-Native Americans. In addition to two autobiographical works, Indian Boyhood (1902) and From the Deep Woods to Civilization (1916), Charles Eastman wrote nine other books, some in collaboration with his wife, Elaine Goodale Eastman (who has told her story in Sister to the Sioux, also a Bison Book).

In The Soul of the Indian, first published in 1911, the author's aim has been "to paint the religious life of the typical American Indian as it was before he knew the white man."