This annual index from the Department of State lists both bilateral and multilateral U.S. treaties and agreements which were in force as of January 1st of each year. Part 1 lists bilateral treaties alphabetically by country and then by subject. Part 2 lists multilateral treaties by subject. This source summarizes over a thousand multilateral agreements and provides information on their status. Unfortunately the most recent supplement only updates status information to January 1,1988.
Indexing is by subject and keyword.
Harvard Law School Library. A comprehensive index to multilateral treaties by subject and country. Unfortunately this volume only covers treaties through December 31,1963. Rohr, Peter H. World Treaty Index. This index provides information about treaties from 1900 to 1980.