The US Constitution
This brilliant document was an example of pure, crystal clarity until, alas, the lawyers and activist jurists got ahold of it. It is now accreted with so much case law and wild twists and turns that many plainly-written articles have been shorn of their original intent.
What US Law Says about the US Constitution
Two web sites provide excellent, authoritative guidance, with searchable text and links to the different articles and amendments. The most valuable feature is the rich commentary on case law that includes scholarly writings on the meaning and interpretations of each section.
You may not agree with where our judges have taken us, but these sites explain how it happened, citing specific court cases.
Findlaw - US Constitution
Cornell Law School - US Constitution
Want more background? Read The Federalist Papers!
I know, they are hard to read for the average layman. I have read various different parts, and it's not easy.
Here's a solution. Go to Founding Fathers.info and click the Federalist Papers link. There you will find a searchable on-line version with a handy index that annotates each paper so you know what it is about.
Founding Fathers - Federalist Papers
More Founding Fathers Resources
Vindicating the Founders - Excellent resource from the Claremont Institute defending the founders from scurrilous lefty charges. Slavery, Women, property ownership... it's all discussed a placed in context.
Restore The Tenth - The Tenth Amendment Coalition