One way to find books (and other materials) that could be primary sources is to search Discover Local (the library's catalog). Try using your topic and one of these keywords:
For example, search these words in the catalog to find memoirs from the Spanish Civil War: Spain personal narratives
Search the Worldcat database to find books held by other libraries. Worldcat is an international catalog of books and other materials owned by thousands of libraries.
You may borrow (interlibrary loan) materials through the Worldcat database. Interlibrary loan is free! But give yourself time for a book to come through the mail.
Please note that occasionally the Worldcat database indicates that the OU library owns an item when Discover Local (our catalog) does not list that item. Usually, the item has been reported lost or missing and is no longer available from our library. Please contact me if you have any questions: firstname.lastname@example.org