There was no definite answer to the question of the most difficult book in the world because different people might have different definition of difficult. However, for some people, the most difficult books in the world might be Dream of the Red Chamber or Capital. 'Dream of the Red Chamber' was a classical Chinese novel that was hailed as a monumental work in the history of Chinese literature. Its complex plot and many characters involved a lot of cultural background and historical knowledge, which may be difficult for ordinary readers to understand. At the same time,'Dream of the Red Chamber' was also a philosophical work that explored human nature, emotions, society, culture, and other aspects, so it might be difficult for some readers to understand. Capital was a work by the German philosopher Karl. It was considered the foundation of modern economics and sociology. Its rich content involved economics, politics, history, and many other fields, so it might be difficult for some readers to understand. Of course, there were other books that could also be considered the most difficult to understand books in the world, such as scientific works such as Relativeness and Quantum Mechanics, or philosophical works such as A Short History of Humanity and A Short History of the Future.