The most difficult book to read, in my opinion, is the King James Bible. It is difficult not only because the prose is outdated, but because the messages are so deep that the reader has to stop to think many times before moving on. It is intentionally vague and unclear in order to spark self reflection.
This makes it difficult to read more than a page or two at one sitting. Other religious texts may also be difficult to read, but because the Bible is so long, so complex, and is a part of history, it has to be the most difficult to read of all books.