George McClellan.
Liberty, Order, and Justice: An Introduction to the Constitutional Principles of American Government, Third Edition.
Indianapolis: Liberty Fund, Inc., 2000.

An excellent introduction to American constitutionalism. Insightful text plus many primary documents. Contains helpful bibliographies for further study.

The Constitution of the United States of America, with Analysis and Interpretation, and Annotations of Cases Decided by the Supreme Court of the United States.

The Avalon Project at Yale Law School: Documents in Law, History, and Diplomacy provides an abundance of primary sources online. These include the first edition of Blackstone's Commentaries and The Federalist Papers.

Reprints of some of the classic texts such as St. George Tucker's 1803 edition of Blackstone's Commentaries (Number 419) and Joseph Story's Commentaries on the Constitution of the United States (Number 412) can be obtained from Lawbook Exchange.