Jonathan Hennessey and Aaron McConnell's The United States Constitution: A Graphic Adaptation is a sweet, quick, thoroughgoing history of the US Constitution. I'm a Canadian and most of my grounding in US Constitutional law began with Schoolhouse Rock — an engaging source to be sure, but sorely lacking in detail. Hennessey and McConnell do great work in picking up where Schoolhouse Rock ends, using a little sprinkling of humor and a lot of illustration to explain such abstractions as the three-fifths compromise, the electoral college, pocket vetos and other critical historical and contemporary elements of constitutional law. The section on the Bill of Rights is especially good in presenting the balanced case for each amendment, debunking the cheap talking-points on both sides of each right's debate.
I only have one criticism of this book, and that is that it stops short of the last eight years' worth of lawmaking and debate, failing to address the Patriot Act, the FISA courts, and other modern constitutional challenges. Still, you'd be hard-pressed to find a better primer for bringing the kids, foreigners and forgetful in your life up to speed on the niceties of the supreme law of the USA so that you can have these discussions.