From the PREFACE:

"...it is one of the very few business organisations of the country whose growth could be traced in trustworthy documents.

There is in existence just such documentary material for a history of the Standard Oil Company as there is for a history of the Civil War or the French Revolution, or any other national episode which has divided men's minds. This has come about largely from the fact that almost constantly since its organisation in 1870 the Standard Oil Company has been under investigation by the Congress of the United States and by the Legislatures of various states in which it has operated, on the suspicion that it was receiving rebates from the railroads and was practising methods in restraint of free trade."

                                -Ida M. Tarbell

note:  The first ever muckraking literary style used in the U.S.  Many writers still use this time honor expose technique!