Searching by "All Fields" in the library catalog is the easiest approach and usually brings up a good number of books; however, it doesn't always bring up the best books for the topic. For that, we need to use a "Subject" search, which allows us to use a little finesse.
Library catalogs use very precise vocabulary to describe subjects, which you can see in the "subject" part of the complete record for a book. For example, In describing a book on mediaeval literature, librarians always use the phrase "Literature, Medieval." Which perhaps seems obvious. However, it becomes less clear when we're looking at the literature for a specific country, which is divided into specific periods. So, for French literature, while we can look for a combination of "Literature Medieval" and "French," a better search would be for the unintuitive French literature to 1500, which will make certain that we get books on the topic of "literature."