The Original Constitution (Robert Natelson)
Easy to read reference book explaining what the Framers meant by each provision in the Constitution
Constitutional Law, 5th ed. (2015) (Erwin Chemerinsky)
Authoritative treatise summarizing constitutional jurisprudence and Supreme Court case law
Miracle at Philadelphia (Catherine Drinker Bowen)
Classic popular history of the Constitutional Convention. Easy to read and very memorable.