FRUS (Foreign Relations of the United States) is a standard source used in researching U.S.foreign policy and diplomatic relations. FRUS is available in many formats, It is included in the legal database Hein Online, in HathiTrust, in printed volumes in the Bobst library stacks at JX 233 .A3, in a nice digital version from the University of Wisconsin, and a growing number as downloadable e-books from the State Department itself.
These are just some of the series you can search in the Libraries' Catalog that may have useful books: