The prime example is Moby Dick, which I have read and I will never recommend.
Here's the catch. For me, classic books need to be readable because I'm not studying literature at university these days. There are many important books published decades or even centuries ago that have great significance, but I'm not going to recommend them. The prime example is Moby Dick, which I have read and I will never recommend. Life's too short and that novel is too hard to read. The most challenging book on this list is The Seven Pillars of Wisdom because of its length.