One effective strategy is to use the "year of publication" facet in the catalog to retrieve sources published during a specific time period. For example, start with a keyword search for "Koreans Japan" and then limit to English and select your date range. Limiting to items published between 1900 and 1940 retrieves periodicals, literary works, government reports, memoirs and more.
Using specific Library of Congress subject terms can help you locate primary sources. Try adding one of the suggested terms to your search, for example:
Japan description and travel
Eastern Question (Far East) and biography
Note that because your research for this class is so specialized, you will probably need to add both language and date range limits as well.