Unless you know the title and/or author of the book you are looking for, research topics should be searched by Keyword to maximize the number of references to a topic.
However, another way to search is to browse by Subject. This strategy is especially useful if you are having trouble narrowing your topic. You may begin with the name of the nation you are studying, for example, "Spain." The name of the nation is followed by the word "History." This broad heading is in turn broken down by historical period, for example, 711-1516.
The Subject heading Spain History 711 1516 yields a list of 23 books.
Books about Europe during the Middle ages are to be found in the D section of the Library. Subclass DA is used for books on the history of Great Britain. Subclass DC covers books pertaining to history of France; subclass DD is devoted to Germany. You an discover the subclass by using the online catalog. Each subclass is further broken down by numbers so as to narrow down to the period of history being studied. For example, DA129-199 covers early British history to 1485.