The Scholarly Monograph continues to be a highly respected and well-recommended source of information for the study of history. A scholarly monograph is a single-volume text written by a scholar in the field. It provides in-depth treatment of a particular problem or topic in the field. Although written primarily for an academic audience, scholarly monographs provide reputable and substantial information for undergraduate researchers as well.
Your instructor is recommending books that are considered "popular" as well. These books are written for a non-scholarly audience. They can be quite useful when researching material that pertains to popular culture. However, coverage of the topic is apt to be more superficial.
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. A keyword search for "Beatles" yielded over 50 search results.
However, another way to search is to browse by Subject. This strategy is especially useful if you are having trouble narrowing your topic.
Books about topics pertaining to The Beatles themselves can be found in the ML section of the library. You may wish to examine other books that deal with rock music. These are also classified as ML.
Since The Beatles' influence was not confined to music, and since they became an international cultural phenomenon, you will find books in many sections of the library that are potentially useful for your research project.