There were many different answers to this question because different people had different meanings and understandings of understanding. However, some books are considered to be one of the most difficult to understand in the world because the ideas and topics they contain are so profound that they require a considerable depth and background knowledge to understand and comprehend. Some of the books that are considered to be one of the most difficult to understand in the world are: Capital-Karl Max: This is a classic work on economics and political economy. Its theme, ideas, and language are very complex and require the reader to have a background knowledge of economics and social sciences to understand. 2. A Short History of Humanity-Yuval Harari: This is a book about human history. It involves many complex concepts and topics such as social systems, technological development, cultural differences, etc. It requires readers to have a deeper understanding of history and civilization. 3 1984-George Orwell: This is a dystopian novel describing the social and political system of a dictatorial country. Its theme and ideas are very complicated and require the reader to have an understanding and thinking ability about politics and society. 4. One Hundred Years of Solitude-Garcia Marquez: This is a magical realism novel describing the family history of seven generations. Its theme and language are very unique, requiring readers to have an understanding and appreciation of literature and language. These books were very complicated and profound, requiring the reader to have considerable depth and background knowledge to understand.