A secondary source is one-step removed from an event and analyzes primary sources.
Examples include a book about World War II that is based on records from the time, or a journal article about Chinese immigrants to Portand.
These databases contain scholarly articles on American history and will be the best places to search for scholarly historical articles about Portland.
Books are essential to historians.
One strategy to find books on your topic is to use a keyword search to find relevant subjects. For example, I searched the keyword phrase chinatown portland in the Portland State Library Catalog (PSU Library selected only).
The seventh result seemed very relevant, an ebook called Sweet Cakes, Long Journey: The Chinatowns of Portland, Oregon.
I clicked on the title, scrolled down to the Details section, then I looked at the subject headings listed. Clicking on a subject heading will get me quickly to many more books on the same topic.
These links will allow you to view theses and dissertations from Portland State University graduates, faculty scholarship about Portland, and dissertations and theses from scholars around the world.