A secondary source is one that was created after the event by someone who wasn't there. Examples include books, journals, magazines and websites. When looking for books you can use ones that you get in physical form or eBooks. Using the tips and link on this page will help you find both types of books.
It isn't one that is secondary in importance!
You will need an SFPL library card and PIN to access these resources. Ask a UHS librarian if you need to get one.
Open source books
Open Library is a free site, although you will need to create an account to be able to download books. Most of these books are older, but it might be worth looking at if your topic is pre-20th century.
Be careful! Not all books in Google Books are full text, but you can use this to check if it's worth requesting from the library.
The best place to look for journals and magazine articles for your research paper is through the UHS library databases (a database is an organized collection of pre-selected resources):
While you're reading your reference article and primary source, keep a list of keywords to search for.
Try broad terms eg Immigration and narrow terms eg Border patrol
Limit the number of terms in a search string. Too many (e.g.Mexican labor on railways 20th century) may not yield much.
Look at the bibliography at the end of the reference article - those might be good sources.