The Heritage Foundation has put on-line its Guide to the Constitution, co-edited by David Forte and Matthew Spalding. This is a line-by-line commentary with major essays by significant legal scholars. Heritage does terrific work with its instant digests on contemporary policy issues, but this is something different, yet relevant to policy debates.
Take this analysis of the first line of Article II of the Constitution, on the nature and scope of executive power, "the vesting clause." There's even a teacher's companion guide, besides the essay by UVA law professor Sai Prakash and a brief (and diverse) bibliography of legal scholarship.
Or consider co-editor Forte's thoughts on the commerce clause, now at the heart of the Obamacare case, to be decided by the Court this term. Are you clear on the meaning of "to ... regulate commerce ... among the several states"? And so it goes, line by line, through the whole Constitution.
The achievement deserves favorable comparison with the best encyclopaedias of legal thought, such as the grand project of the late Leonard Levy. And besides Heritage's is on-line, will be constantly updated (not a living Constitution, but a lively commentary) and free.