The Gale Virtual Reference Library contains hundreds of reference e-books, including specialized history encyclopedias with entries on many U.S. History topics.
Search for a topic, then use "Search Within" from your results page to limit to more specific topic.
Or choose a specific Encyclopedia, then use "Search Within Publication" to find specific topic.
The search box default is "basic search". This is a keyword search and often the best search. You can also change the drop down menu to "subject", for fewer results but sometimes better results.
Look at "Filter Your Results" on right side of results page. Try Filtering by Subject and Document Type. The "excerpt" document type can include primary sources, sometimes excerpts, and sometimes the entire document.
Also look at "Search Within" to find a more specific topic within your search results.