Start with a keyword search in the catalog. Once you have located a relevant item, you can look at the Subject within the full record tab and follow those links to find related books.
You can also refine your results by using the facets on the left side of the screen. If you are looking for online sources only, narrow down your search by clicking the box next to the "available online" facet.
To find primary resources, you can add any of the following keywords to your search: letters, diaries, speeches, newspaper articles, autobiographies, oral histories, government and organizational records, statistical data, maps, photographs, motion pictures, sound recordings, advertisements, artifacts, etc.