Old Tax Acts and Regulations: In the early 2000s, I converted the old income tax acts to searchable text and scanned the old regulations into PDFs images.  These acts and regulations are linked here and explained.

"In Lieu Of": There is a string of old tax acts and regulations, ultimately replaced by the 1954 IR Code and 1986 IR Code. Most do not know how these various acts are interrelated, which is explained at this link.

What Is Income: An analysis of the various "income" provisions of the old tax acts is posted here.

Employment Taxes: The old tax acts provided for withholding of the income tax. Learning about these taxes is important, as explained at this link.

The Adoption of Constitutional Amendments: When Bill Benson was doing his research regarding the non-ratification of the 16th Amendment, I was engaged in the legal research of this question. This file is a short synopsis of the relevant law with links to some good cases.

Quick Link: The above brief contains cites and quotes of cases relevant for this issue. If you wish to just read some of these cases, this file has the links to those which are posted to this site.
Uncertainty of the Law: Surprisingly, American courts have failed to identify what is the nature of the income tax. This uncertainty of the constitutional classification of this form of taxation presents a monumental due process problem for the American people. Members of Congress should be informed of this uncertainty of the law which they did not create.

Lloyd Long Transcript: Lloyd Long was a farmer indicted for tax crimes in Chattanooga, but hard work resulted in an acquittal. This is his trial transcript.

Vernie Kuglin Transcript: Vernie was a FedEx pilot indicted for tax crimes in Memphis. She worked on her case and was acquitted at trial. This is her trial transcript.

Tommy Cryer Transcript: Tommy, a Louisiana lawyer, was indicted for tax crimes and was tried in Shreveport, his hometown. Being persuasive, he naturally was acquitted.

Creation of the IRS: Where is the statute which created either the Bureau of Internal Revenue or the Internal Revenue Service? This is a handy letter which can be used to make such an inquiry. Want to see how the IRS keeps its records? Read this GAO Report.

Ruml Speech: Beardsley Ruml was chairman of the New York Federal Reserve Bank during World War II. He gave an amazing speech to the American Bar Association in 1945 which explained the operation of our monetary and tax systems in a most enlightening way. His speech was published in a periodical named American Affairs in its January, 1946, issue, which is reproduced here as a PDF file.

1921 Annual Report of Treasury Secretary: Here are excerpts from a 1921 government report stating that high taxes inflict harm.

Biblical Observations: The Bible contains some interesting comments regarding the subjects of taxation by the "kings of the earth."

Joe Banister: Joe Banister is a former IRS Special Agent who quit after asking his superiors questions about the validity and application of the federal income tax. He was later indicted but acquitted at trial. This is his website.

Phil Hart: Phil is an engineer whose office is located in beautiful Coeur d'Alene, Idaho. He engaged in a very detailed study of the meaning of "income" and wrote his book, Constitutional Income: Do You Have Any? This is his website.