I rarely quit books. I can think of only two cases where I quit reading books because of the books themselves. The first, as I mentioned in another recent thread, was Dee Brown’s Bury My Heart at Wounded Knee, which I quit a quarter of the way into it because it was too dull. The second was Charlotte Bronte’s Villette, which I quit when I was about 14 because I was having trouble understanding it - as I recall, a good deal of the dialogue is in French without a translation provided. I may come back to it one day, but not soon.
The second category is books that I haven’t finished reading because something else came up. This usually applies to books that I started reading at the end of the summer and which I had to put down once school began. Once I finally have time to return to them, I’ve been out of the book too long to return to it. This was the case with George Orwell’s 1984, Ken Kesey’s One Flew Over the Cuckoo’s Nest and Emily Bronte’s Wuthering Heights, all of which I enjoyed (though WH is confusing!) but just didn’t have time for.