Let me say up front that I'm a rather intellectual reader. My personal shelf includes The Art of War, The Lord of the Rings, and the Hitchhiker's Guide to the Galaxy. So now that you know where I'm coming from, it'll make my point all the clearer.
Herman Melville's Moby Dick.
Saw the movie (the one with Patrick Stewart) in an english class and decided to read the book, and GOOD GOD! It's like Herman Melville got sidetracked by anything! I'm reading along and all of a sudden, in the middle of the action, he goes off on this historical tangent for like twenty pages!
I got about halfway through, then finally got sick of it and gave up.