An Overview is a book which addresses a time period such as The Cambridge History of China. Use the index to find your topic; it may be a chapter or just a few pages. An overview is meant to place your topic within the context of the time and larger world.
A Monograph is a book which addresses your specific topic. If you are researching a person, a monograph could be a biography of the person. If you are researching an event, a monograph might be The Opium War, 1840-1842.