Personally, I see the Federalist Papers (which were written by Alexander Hamilton, James Madison, and John Jay) as the most complete expression of what the Constitution (and the Bill of Rights) means and why each article and set of powers was allocated the way it was. All three men are exceedingly clear and concise about the reasoning behind why the Constitution is the way it was, if you're willing to parse a tiny bit to compensate for the 1700s language. All three men were federalists, meaning they were 'pro-Constitution', and they were basically trying to argue their case to the public for why the United States of America should adopt the Constitution (as opposed to the anti-Federalists, who were either pro-'fixing the Articles of Confederation' or just generally anti-'adopting the Constitution' for various reasons). There are several editions of the Papers on Amazon; I'd recommend an annotated version if you want a little bit more of the history, background, and context behind certain things in the text. Taken together with the lesser-known but just as important Anti-Federalist papers, they represent frankly an amazing breadth and depth of exposition on the Constitution, its benefits, and its possible pitfalls.

As far as scholarly texts go, though, I recommend Akhil Reed Amar's America's Constitution and Gordon Wood's The Creation of the American Republic. For an easy-to-read/digest layman's version, Fault Lines in the Constitution: The Framers, Their Fights, and the Flaws that Affect Us Today is a really accessible book that was written by a husband and wife team (he a Constitutional Law scholar, she a children's book author). It was technically written for middle school students but provides a thoroughly excellent overview of the Constitution, how it came to be, and why it says the things it does.

To go along with what /u/erissays said, if you were interested exclusively in the legal opinions on the constitution and their evolution you could check out Erwin Chemerinsky’s Constutional Law: Principles and Policies. In legal circles he’s considered the preeminent source for Constitutional law and his books are required reading at many if not most law schools.

There are some facets of constitutional law relating to the courts that will not be covered in depth there, namely Jurisdiction and the Erie doctrine(which explains the nuances of the evolution of judicial power sharing between the state and federal courts). For a background on those I recommend Thomas Rowe’s Civil Procedure. Your local law library should have copies of both if you live next to a large town in the States.