The ORO has vast numbers of encyclopedia entries; look for ways to limit what the search engine brings you. As a first brush, limit the range of titles you search by using the 'Search within my subject specialization' limiter. The box is in the upper right corner of Oxford Reference Online's home page.
After you've done a search, you can use the left hand filtering to limit further. Be sure to choose the 'unlocked' option under 'By Availability'.
The following two Oxford encyclopedias may be of interest for your assignment. You can search within each of these encyclopedias (rather than the whole of ORO) by using the little, hard to notice, search box in the left hand navigation:
And finally, the Library has hundreds of encyclopedias on all manner of historical topics. A list of those is here.
Books in libraries are shelved by their call numbers which correspond to topics. In theory this locates books on the same topic in the same general vicinity of the library collection. PSU Library and most academic libraries use the Library of Congress Classification system. (A full outline of the topics and call numbers is here.) You can physically scan the shelves in these call number ranges for works on your topics.
On the 2nd floor in the Reference collection, use these call numbers to navigate to encyclopedias and other reference works on your place and period of history. Note the arrangement is first by place and then by period. If you want to browse in the regular book stacks, consult the outline to get a more precise call number range to browse in.
D History (General)
DA Great Britain
DAW Central Europe
DB Austria - Liechtenstein - Hungary - Czechoslovakia
DC France - Andorra - Monaco
DE Greco-Roman World
DG Italy - Malta
DH Low Countries - Benelux Countries
DJ Netherlands (Holland)
DJK Eastern Europe (General)
DK Russia. Soviet Union. Former Soviet Republics - Poland
DL Northern Europe. Scandinavia
DP Spain - Portugal
DR Balkan Peninsula
DU Oceania (South Seas)
E-F - History of the Americas
E 11-143 America
E 151-909 United States
F 1-975 United States local history
F 1001-1145.2 British America (including Canada)
F 1170 French America
F 1201-3799 Latin America. Spanish America
Often a very simple keyword search combined with the library catalog's subject heading filters can get you a perfect list of books. Your search will usually start with a natural language keyword, but pay attention to the controlled vocabulary of the catalog. When you find promising titles, use the subject headings that have been assigned to that title to find more works of interest.
In the following example, I want to find books that address the history of the 1920's in the United States.
The search starts with the keywords 1920's history using the PSU Library scope.
Next using the filters on the left side bar, the results are limited by Resource Types 'books' and 'ebooks' and by the subject headings 'United States', '1900-1999', 'Economic conditions',and 'Social Conditions'.
In reviewing the list of books, I find a book that looks perfect for my purposes, Change and Continuity in Twentieth Century America: The 1920's. By clicking on the book title, I get a more complete description of the book including subject headings which include a heading: 'United States-History 1919 - 1933'.
Notice that the heading is a hyperlink and by clicking on it, I can retrieve all the books the Library has on the topic.