We've just received A Companion to the United States Constitution and Its Amendments, which provides explanations, historical background, and case citations to the U.S. Constitution, Bill of Rights, and subsequent Amendments.
For example, when discussing the sixth amendment, author John R. Vile points out that while the Sixth Amendment provides for a speedy and public trial, the U.S. Supreme Court declined to precisely define what speedy meant. After the case (Barker v. Wingo) Congress passed the Speedy Trial Act of 1974, providing for precise time frames for a "speedy trial" that the federal government must follow. If you're interested in the original text only, the appendix material includes the full text of the U.S. Constitution, Amendments, the Declaration of Independence, and the Articles of Confederation.
In addition to providing explanations for individual sections of the Constitution, Bill of Rights, and Amendments, Vile includes 50 leading U.S. Supreme Court cases interpreting the Constitution; a glossary; highlights of Constitutional history; and a list of U.S. Supreme Court justices.