"The Devil Soldier" is a historical account of the life of John E. McKinney, an American who became a soldier in the Chinese Imperial Army during the late 19th century. He is known for his fierce reputation and involvement in the Boxer Rebellion, a violent uprising against foreign influence in China. The book explores themes of identity, loyalty, and the complexities of war. It highlights McKinney's transformation from a soldier to a mercenary, as well as his interactions with various cultures and the moral dilemmas he faced during his military career.