Unless you know the title and/or author of the book you are looking for, history topics should be searched by Keywordto maximize the number of references to a topic.
The word "sources" appearing next to an 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," "personal narrative" etc. to your keyword search term.
Shadow: Five Presidents and the Legacy of Watergate by 
Watergate: The Presidential Scandal That Shook America by 