Which is the hardest book you have ever had to read? and why?

Virginia Woolf's To the Lighthouse and The Waves. The two novels carry a very complicated style of writing; the setting is very much ambiguous; the characters are few but, relying on the stream of consciouness, Woolf manages to manipulatethe notion of time as to ender them the way she wants.

a book has been written ages ago , as I dont find the modern words used these days ,only main and raw arabic words  which makes the text challenging .