The International Court of Justice (ICJ) identifies the sources of international law in the ICJ Statute, Article 38 which reads:
1. The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:
a. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
b. international custom, as evidence of a general practice accepted as law;
c. the general principles of law recognized by civilized nations;
d. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
2. This provision shall not prejudice the power of the Court to decide a case ex aequo et bono, if the parties agree thereto.
The primary sources you will use are:
Excellent descriptions and analysis of the Sources of International Law:
The Oxford Handbook on the Sources of International Law / edited by Samantha Besson and Jean d'Aspremont (2017) - 5th Stacks (call number KZ1275.5 .O94 2017)
The Sources of International Law / Hugh Thirlway (2014) - 5th Stacks (call number KZ1279 .T45 2014)
On the Foundations and Sources of International Law / edited by Ige F. Dekker and Harry H.G. Post (2003) - 5th Stacks (call number KZ3410 .O5 2003)
Sources of International Law / edited by Martti Koskenniemi (2000) - 5th Stacks (call number KZ3410 .S67 2000)
Sources of International Law / V.D. Degan (1997) - 5th Stacks (call number KZ1242 .D43 1997)
Rethinking the Sources of International Law / by G.J.H. van Hoof (1983) - 5th Stacks (call number KZ1242 .H66 1983)
Sources of Customary International Humanitarian Law: