Rudolf L. Bindschedler, "International Organizations, General Aspects" in 2 Encyclopedia of Public International Law 1289 (Rudolf Bernhardt, ed. 1995) provides a detailed discussion of the legal basis, structure and function of IGOs.
Encyclopedia of Associations: International Organizations (Grant J. Eldridge, ed.) includes extensive listings of organizations, their stated purpose, publications, addresses, etc. There is an electronic version called Associations Unlimited , available by subscription.
Volume 1 of the Europa World Year Book contains information on various international organizations, including address, phone and fax numbers.
International Agencies and Information on the Web provides access to many websites that collect information about international organizations, both intergovernmental (IGO) and non-governmental organizations (NGO).
Giuseppe Schiavone, International Organizations: A Dictionary and Directory (6th ed. 2005) contains directory information for each organization listed along with an overview of its objectives, structure and activities. Membership tables and a list of foundation dates for the organizations covered are also included.
Yearbook of International Organizations includes acronyms and descriptions of each organization, addresses, telephone and fax numbers, as well as detailed information about the organization's structure, purpose and activities. Links to selected international organizations from the Yearbook are available online.