Primary means written at or near the time of the historical event.
Secondary means written many years later, probably by a scholar researching the historical event.
Books are organized in the library by the Library of Congress Classification System. LC Subject Headings are assigned to every book. They will help you locate more relevant books on your topic.
Oxford University Press presents a new podcast series commemorating the centennial of the United States entering World War One. Each episode is hosted by Thomas Zeiler and features engaging dialogue from a variety of Oxford University Press authors.