Restatement of the Law (Third): The Foreign Relations Law of the United States: KF395.A2 F76 1987
This version of the restatement is widely recognized source of customary international law from the perspective of the United States.
U.S. Department of State Website: www.state.gov
The main website for the U.S. Dept. of State is chock full of information regarding U.S. foreign policy and practice. The site can be browsed by countries, regions, topics, or sub-agencies. There is also a search box in the top-right corner that allows for basic keyword searching of the whole site.
Digest of United States Practice in International Law by U.S. Dept. of State: Online and KZ21.R68
This annual digest is issued by the Department of State to outline U.S. practices and understandings regarding international law. It is available in print at the library dating back to 1974 and online dating back to 1989.
United States Practice in International Law by Sean D. Murphy: KF4581.M87 2002
This two-volume set covers U.S. foreign practices from 1999-2004 in a number of areas, including economics, the environment, criminal law, and human rights. Volume 1 covers 1999-2001 and Volume 2 covers 2002-2004.