Much more so than in American law, in international law much of the best scholarship appears in books.  There are books reviewing the most important sources, issues and policy debates on virtually every topic.  You are strongly urged to find both “classic” books and the most recent books on each issue you research.  Unlike articles, which often focus on a narrow legal issue or specific controversy, these books have a general focus and review the entire area of law systematically.  So for doctrinal areas such as the law of treaties, state responsibility, the status of international organizations and others, the best sources are books.