Most gruelling book you have read?

Something you were really struggling with but your willpower kept you reading.

Finnegans Wake

I read it simply because I wanted to put my money where my mouth was. It was a difficult task; it was nonsensical; it was stupid. In the end, however, I was able to finish it and that at least gave me leverage in declaring it personally my worst book of all time. I needed to wash my brain afterward with Faulkner. It was just so bad.