Search the online library catalog for your topic, then try adding one of the following words to your search to find collections of primary sources:
"documents," "sources," "letters," or "diaries"
e.g., "Korean War AND Sources" or "Civil War AND Diaries"
Don't forget to change the "Held By" selection to Libraries Worldwide to find books you might be able to obtain through interlibrary loan.
One way to find more books on your topic is to follow the subject headings of a book you have found to be appropriate for your topic. The Library of Congress Subject Headings is list of subjects that this library uses to categorize books on a particular topic.
For example, let's say you found a good book on your topic "nurses serving in the Vietnam War." That book's record in the library catalog has the Subjects:
You can click on any of these similar subjects to find out what other books are available on these subjects. And even though they are related topics, each one of these may have a different list of books with that subject heading, so look at more than one of the subjects to get the best results.