The above answer is mostly correct. However, charters of international organizations would not be considered a material (subsidiary) source. Charters are basically multilateral treaties in that they regulate inter-state affairs. Such charters can carry binding force and be reflective of customary laws and jus cogens, though not explicitly. Thus, each charter or similar documents would have to be evaluated in its own context to determine its categorization as a formal or material source, and consider questions regarding its binding force, and nature, whether it is a law-making charter (treaty) or not.

A great source for studying and understanding international law is the book International Law by Malcolm N. Shaw. Its pdf is easily available online.