US Digests of State Practice in International Law
Cadwalader, John L. Digest of the Published Opinions of the Attorneys-General, and the Leading, Decisions of the Federal Courts, With Reference to International Law, Treaties and Kindred Subjects (1877)
Wharton, Francis, ed. A Digest of the International Law of the United States Taken from Documents Issued by Presidents and Secretaries of State and from Decisions of Federal Courts and Opinions (1887)
Moore, John Bassett, Digest of International Law as Embodied in Diplomatic Discussions, Treaties and other International Agreements, International Awards, the Decisions of Municipal Courts, and the Writings of Jurists, and Especially in Documents, Published and Unpublished, Issued by Presidents and Secretaries of State of the United States, the Opinions of the Attorneys-General, and the Decisions of Courts, Federal and State (1906)
Hackworth, Green Haywood, Digest of International Law (covers 1906-1939)
Whiteman, Marjorie M., Digest of International Law (covers 1940-1960, but varies by topic)
Digest of United States Practice in International Law (covers 1973-1980)
Prepared by: Arthur W. Rovine, 1973-1974; Eleanor C. McDowell, 1975-1976; John A. Boyd, 1977; Marian L. Nash, 1978-1980.
Cumulative Digest of United States Practice in International Law (Covers 1981-1988)
Digest of United States Practice in International Law (covers 1989-2017+)
American Journal of International Law
Each issue (beginning in 1959) includes a feature "Contemporary Practice of the United States relating to International Law"
Other sources of US Practice in International Law
Hyde, Charles C., International Law: Chiefly as Interpreted and Applied by the United States KZ4113 .H93 1945
Murphy, Sean D., United States Practice in International Law KF4581 .M87 2002
Restatement, Third, Foreign Relations Law of the United State KF4651 .R47 1987 (Ref); Also HeinOnline, Westlaw* and Lexis*
See also:
Roberts, Anthea Elizabeth
Traditional and Modern Approaches to Customary International Law: A Reconciliation
95 Am. J. Int'l L. 757 (2001)
*Westlaw/Lexis access restricted to law students only