Gen Mattis is an expert on the Plains Indians, and part of of his vast library includes hundreds of books on the various tribes.
He’s a very interesting man:
Early life and education
Mattis was born in Pullman, Washington on September 8, 1950.[3] He graduated from Columbia High School, Richland, Washington, in 1968 and enlisted in the United States Marine Corps in 1969. He later earned a Bachelor of Arts degree in history from Central Washington University and was commissioned a second lieutenant through ROTC on January 1, 1972.
Mattis during his service years was considered something of an intellectual among the upper ranks, with his personal library numbering more than a thousand volumes. Major General Robert H. Scales (ret.)(PhD), described him as “....one of the most urbane and polished men I have known.” Reinforcing this intellectual persona was the fact that he carried on his person a copy of the Meditations of Marcus Aurelius throughout his deployments.
Source: Wikipedia