Reference works make excellent starting points for research; they provide concise articles on key topics by scholars of the field, often accompanied by suggestions for further reading. The following are available online through the Penn Library website, and a number also can be found in printed versions, usually in Van Pelt Reference.
The sources listed below are print volumes shelved in Van Pelt Library Reference Stacks, on the first floor of Van Pelt. This is only a sample, of course; try to find more with Franklin keyword searches, such as encyclopedias international relations.