Books that are primary sources can be either books published in the time period of interest or collections of primary source materials published as a book (online and physical).
Unfortunately, there is no precise and comprehensive way to search for primary source materials in the library catalog. There are some subject headings that are more likely to be primary sources, such as:
Women--Europe--History--diaries
Women--Europe--History--Sources
Other terms that you can add to topic-specific keywords:
Letters
Documentary history
Documents
Sourcebook
Correspondence
American Eras: Primary Sources
American Eras: Primary Sources
Human and Civil Rights: Primary Sources
Understanding U. S. Military Conflicts Through Primary Sources
