The 10th Amendment is a tautology only in so far as we take it for granted. But it serves a very important purpose if and when new issues arise that had not previously been anticipated by the Founders.
It helps to think of the Constitution as a set of operating instructions for the machine that is the federal government. If something isn't explicitly enumerated, the machine needs a way of knowing how to render a decision. The 10th Amendment intentionally limits the decisionmaking capabilities of the machine to the expressly listed operating instructions, and no more.
"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
In plain English: "Anything we didn't mention in the Constitution is the domain of the states."
It's important to point this out explicitly, so that the federal government does not make assumptions of power if/when it encounters an issue not specifically enumerated in the Constitution.
That is what the text of the amendment says, and contemporary sources make clear that the framers included it in the Bill of Rights to make legally explicit the limited scope of federal power that they all had in mind. This is the explanation given by James Madison in the Federalist Papers, No. 45 [0], for example.
It was only a decade or two into the twentieth century that judges began to call the Tenth Amendment a meaningless "truism". It's no surprise that these were the same judges who first lent their imprimatur to vast new federal powers, mostly through boundlessly expansive interpretations of the commerce clause and especially of the general welfare clause relating to taxation.
I really wouldn't use the Federalist Papers to justify Constitutional intent, or even Madison's true opinion on any subject he wrote within. He wrote them because though he thought the Constitution as written would fail, it was the best they could do at the time and would hold the Union together until it could be changed, which it was.
More importantly, Judicial Review was created and filled the gap Madison saw between the powers enumerated in the Constitution and their implementation.
Within the mainstream of jurisprudence, as well as good case law, the Federalist Papers are absolutely used to clarify and explicate the original meaning of the Constitution. They are far and away the leading source for that purpose.
If you're interested in how well the Papers do (or don't) reflect the intent of the framers and contemporary understanding of the Constitution, I can suggest a 40-ish page law review article [0] by Gregory Maggs. It also covers the history of the Papers' citation in legal scholarship on constitutional interpretation and in decisions of the Supreme Court.
That's really interesting. I'm not a lawyer, just someone who did his thesis on Madison, specifically why the Constitution didn't fail in the way he thought it would; I can only speak to that. I'll read the PDF, thanks.
If you want, the Library of Congress has copies of his correspondence and notes, they're a really interesting read. You can also find them at http://oll.libertyfund.org/index.php
Pash is right, but I think he overstates his case a bit. It's important to realize that there are as many original interpretations of the Constitution as there are framers, and that the Federalist Papers represents the views of a certain subset of the people who wrote the Constitution. They are a popular reference, but they are by no means gospel.
It helps to think of the Constitution as a set of operating instructions for the machine that is the federal government. If something isn't explicitly enumerated, the machine needs a way of knowing how to render a decision. The 10th Amendment intentionally limits the decisionmaking capabilities of the machine to the expressly listed operating instructions, and no more.