3. United Nations

1. General
2. Treaties and Related Documents
3. Travaux préparatoires
4. Commentaries
5. Practice
6. Nongovernmental Organizations, Private Research Centers: Guides to U.N. Research by Non-U.N. Entities

1. General

Website of the United Nations: http://www.un.org/

Principal Organs and Specialized Agencies: http://www.un.org/en/aboutun/structure/index.shtml
UN at a Glance: http://www.un.org/en/aboutun/index.shtml
Audiovisual Library of International Law Historic Archives International Organizations Charter of the United Nations, 1945 http://untreaty.un.org/cod/avl/ha/cun/cun.html

2. Treaties and Related Documents

Charter of the United Nations: http://www.un.org/en/documents/charter/index.shtml

3. Travaux préparatoires

3.1 Atlantic Charter, 14 August 1941: http://avalon.law.yale.edu/wwii/atlantic.asp

3.2 Declaration by the United Nations (Joint Declaration by the United States, the United Kingdom, the Union of Soviet Socialist Republics, China, Australia, Belgium, Canada, Costa Rica, Cuba, Czechoslovakia, Dominican Republic, El Salvador, Greece, Guatemala, Haiti, Honduras, India, Luxembourg, Netherlands, New Zealand, Nicaragua, Norway, Panama, Poland, South Africa and Yugoslavia), 1 January 1942: http://avalon.law.yale.edu/20th_century/decade03.asp

3.3 Joint Four-Nation Declaration (the United States, the United Kingdom, the USSR and China), 1 November 1943: http://avalon.law.yale.edu/wwii/moscow.asp

3.4 Declaration of the Three Powers, Tehran, 1 December 1943: http://avalon.law.yale.edu/wwii/tehran.asp

3.5 Proposals for the Establishment of a General International Organization, 9 October 1944: http://www.udhr.org/history/dumbarto.htm

3.6 Protocol of Proceedings of the Crimea Conference, Yalta, 11 February 1945: http://avalon.law.yale.edu/wwii/yalta.asp

3.7 Documents of the United Nations Conference on International Organization: v. 1-2. General; v. 3. Dumbarton Oaks proposals: Comments and Proposed Amendments, English text; v. 4. Propositions de Dumbarton Oaks: commentaires et projets d'amendements, texte franais, etc.; v. 5. Delegation, Chairmen, Steering Committee, Executive Committee; v. 6-7. Commission I, General Provisions; v. 8-10. Commission II, General Assembly; v. 11-12. Commission III, Security Council; v. 13. Commission IV, Judicial Organization; v. 14. United Nations Committee of Jurists; v. 15. Coordination Committee, the Charter, Lists; v. 16. Index; v. 17-18. Documents of the Coordination Committee, including Documents of the Advisory Committee of Jurists; v. 19-20. Documents du Comite de cordination, comprenant les documents du Comite consultatif de juristes; v. 21. General index; v. 22. Index general.

4. Commentaries

Bruno Simma et al., The Charter of the United Nations: A Commentary, 2nd ed., Oxford University Press, 2002, 2 v.
Leland M. Goodrich, Edvard Hambro and Anne Patricia Simons eds., Charter of the United Nations: Commentary and Documents, 3d and rev. ed., Columbia University Press, 1969.
Jean-Pierre Cot et Alain Pellet eds., La Charte des Nations Unies: commentaire, article par article, 3e ed., Economica, 2005, 2 v.

5. Practice

Research Guide: http://www.un.org/Depts/dhl/resguide/
U.N. Documentation Centre: http://www.un.org/en/documents/
- This is the most useful search tool, integrating the followings:

ODS: Official documents of the United Nations: http://documents.un.org/welcome.asp?language=E
UN-I-QUE: United Nations Info Quest: http://lib-unique.un.org/lib/unique.nsf
UNBIS-Net: United Nations Bibliographic System: http://unbisnet.un.org/

For specific cases, see:

Yearbook of the United Nations: http://unyearbook.un.org/
Repertory of Practice of United Nations Organs: http://www.un.org/law/repertory/
Repertoire of the Practice of the Security Council: http://www.un.org/en/sc/repertoire/
United Nations Juridical Yearbook: http://www.un.org/law/UNJuridicalYearbook/index.htm

UN practice on international law

International Law Website: http://www.un.org/en/law/index.shtml
Codification and Progressive Development of International Law: http://www.un.org/law/lindex.htm
Sixth Committee (Legal): http://www.un.org/ga/sixth/
Office of Legal Affairs: http://untreaty.un.org/ola/
International Law Commission: http://www.un.org/law/ilc/index.html

6. Nongovernmental Organizations, Private Research Centers: Guides to U.N. Research by Non-U.N. Entities

Globalex: Researching the United Nations: Finding the Organization's Internal Resource Trails: http://www.nyulawglobal.org/globalex/United_Nations_Research1.htm
Columbia Law School Library: http://library.law.columbia.edu/guides/United_Nations

Columbia Law Library: Research Guide: The United Nations: http://www.law.columbia.edu/library/Research_Guides/internat_law/un

American Society of International Law: http://www.asil.org/un1.cfm
Yale University Library: http://www.library.yale.edu/govdocs/un.html

Yale University: United Nations Scholars' Workstation: http://www.library.yale.edu/un/index.html

New York University School of Law Library: http://www.law.nyu.edu/library/research/researchguides/unitednationsresearch/index.htm
Duke Law Library: Research Guides: United Nations: http://www.law.duke.edu/lib/researchguides/un.html
Stanford University Libraries: United Nations Documents and Publications Guide: http://library.stanford.edu/depts/jonsson/collections/intl/un.html