Unless you know the title and/or author of the book you are looking for, research topics should be searched by Keyword to maximize the number of references to a topic.
However, another way to search is to browse by Subject. This strategy is especially useful if you are having trouble narrowing your topic.
The word "sources" appearing next to a subject entry is usually a good indication that the text is at least in part a primary source. You can increase your chances of turning up primary sources by adding terms such as "sources," "letters," "diaries," etc. to your keyword search term.