Customary International Law
Rules and practice of international law followed by states:
ICRC Customary International Humanitarian Law: rules and national practice of customary international humanitarian law, with summaries of rules and citations to treaties, cases, and military manuals.
Restatement of Foreign Relations Law of the United States 3d & 4th (Hein Online ALI Library, Lexis, and Westlaw): Analysis of international law and foreign policy of the United States, with statements on customary international law. Draft 4th edition volumes are on Treaties, Sovereign Immunity, and Jurisdiction. Sections include black letter discussion of the law, a comment, and reporter's notes with case citations.
Treatises and journal articles can be cited as statements of custom and practice, especially classics and works by noted authors. A country's treaty practice can also be evidence of customary international law.