There is no definite answer to this question because the most difficult books in the world can be measured by different standards. In some cases, people might think that science fiction novels such as A Short History of Humanity or The Three-Body Problem are difficult to understand because they involve a lot of scientific knowledge and philosophical thinking. They require readers to have a certain background knowledge to understand the ideas and perspectives in them. In other cases, some ancient classical literary works may also be considered difficult to understand, such as The Analects of Confucius, Tao Te Ching, etc., because they contain profound philosophical thinking and emotional expression, readers need to have a certain cultural background and philosophical accomplishment to understand and understand the deep meaning. Therefore, there is no clear answer to this question. Different readers may have different answers.