The library uses the Library of Congress classification system, which organizes books according to subject matter. In the library stacks, Library of Congress call numbers can make serendipitous discovery of other books on your topic possible-- finding one book on the shelf, you'll see more on the same topic on neighboring shelves.
In online research, you can use Library of Congress subject headings to lead you to other books on your topic. Here is a short video showing how to use subject headings from one book to find more books on your topic.
Google Books is a great option when you're not finding what you want in the library's book catalog. It sometimes even has the full text of the books it finds!
JZ5-6530 International relations
JZ24-38 Societies, associations, academies, institutes, etc., for the study of international relations
JZ221-1153 By region or country
JZ1249-1254 Relation to other disciplines and topics
JZ1305-2060 Scope of international relations. Political theory. Diplomacy
JZ1329.5-1395 By period
JZ1400-1454 Diplomatic and consular service
JZ1464-2060 Scope of international relations with regard to countries, territories, regions, etc.
JZ3674-3875 State territory and its parts
JZ3685 Boundaries
JZ3686-3875 International waters
JZ4835-5490 International organizations and associations
JZ4841-4848 Political non-governmental organizations. NGOs
JZ4850-5490 Intergovernmental organizations. IGOs
JZ4853-4934 League of Nations
JZ4935-5160 United Nations
JZ5511.2-6300 Promotion of peace. Peaceful change
JZ5514-5526 Societies, associations, academies, institutes, etc., for peace promotion, research and education
JZ5587-6009 International security. Disarmament. Global survival
JZ6010-6060 Pacific settlement of international disputes
JZ6360-6377 Non-military coercion
JZ6385-6405 The armed conflict. War and order
JZ6422-6422.5 Neutrality. Non-participation in wars. Norms of neutrality
JZ6530 Humanitarian aspects of war
KZ2-6795 Law of nations