Which "robber baron" did Ida Tarbell write about?

The Robber Baron was John D. Rockefeller. The journalist and writer Ida Tarbell, in her book The History of the Standard Oil Company, publicly exposed in 1904, all the ruthless strategies and ways in which John D. Rockefeller, achieved that his company, Standard Oil became the largest monopoly of the world (at that time). Ida Tarbell's book inspired Americans to change their way of seeing the economy, and fight against monopolies, like Rockefeller's.