One of the most important Supreme Court cases on the federal income tax was Brushaber v. Union Pacific Railroad,  240 U.S. 1 (1916). See Wikipedia article. Larry Becraft comments on the case in his article, "Uncertainties of the Income Tax". It is also discussed by Jeff Dickstein in his book, Judicial Tyranny and Your Income Tax.

All the key files are now online as PDF's here at the links below:

• Brushaber Petition — Provides insight into the original arguments.
• Brushaber brief — Arguments made to Supreme Court.
• Union Pacific brief — Arguments made to Supreme Court.
• United States brief — Arguments made to Supreme Court, and the ones the Court sustained.
• Brushaber Transcript — Arguments made to Supreme Court.