Books are shelved in general subject categories using the Library of Congress classification system. You may want to supplement your use of the catalog with browsing shelf areas for your topic. Below is a brief listing of some of the subject locations in the field of Spanish.
If you look to the left of search results, you will see multiple ways to refine a search - by language, by how recently it was published, by format (print book versus article). You can also expand your search to libraries worldwide and then request books from another library. In this example, the search was limited to books published in the past ten years in libraries worldwide.
Catalogers use a consistent list of terms to describe the subject of books. They may not be the words you originally type in. For example, books about Spanish cinema (or film) have the subject heading "Motion Pictures - Spain." Play around with your search terms. If you want to see what catalogers use for subject headings click on the title of a book and then on "description."
Yes! It takes a few days, but if you want a book from another library, use Interlibrary Loan. If you change the default library search to "libraries worldwide" you'll see a "request" button that can be used to ask for a book to be sent from another library. You'll get an email when the book arrives. Think of it as Amazon without the bills. (You do have to give the books back, though.)
To find out if a particular person or topic is mentioned in a book (or if you want to peek inside a particular book to see if it might be usefu, try Google Books, Hathi Trust, or Amazon (though to search inside books on Amazon you may have to have an account). In some cases, but certainly not all, the full text of books have been scanned and are searchable, though you will not be able to print out or in some cases even see more than a few words surrounding your search results. However, this might help you decide whether to order a book through interlibrary loan.