‘The term “international organization” refers to an organization established by a treaty or other instrument governed by international law and possessing its own international legal personality. International organizations may include as members, in addition to States, other entities’ (Art. 2 lit. a DARIO, Yearbook of the International Law Commission [2011] vol. II, Part 2).
Printed materials
Internat 680 is the section dealing specifically with international organisations, but searches of SOLO may well uncover useful titles in other sections.
The specific subject search for SOLO is (annoyingly) International agencies
In MEPIL, start with article "International Organizations or Institutions, General Aspects" - and work out from there via the links etc
Principles of the Institutional Law of International Organizations by 
Judicial Decisions on the Law of International Organizations by 
Research Handbook on the Law of International Organizations by See further the guide written by our colleague in the Bodleian Official Papers
Status of NGOs in International Humanitarian Law by 
Intellectual property, human rights and development : the role of NGOs and social movements by