Yeah that was a toughie. WifeofZeugma didn't make it through that one, and that's saying something. The playing card cypher was cool.
Any list of unreadable books mustinclude Dianetics. Granted, it's not supposed to be considered fiction, but still, it's the worst thing I've ever read that comes to mind.
Interesting choice. I've never read it, but at or near the top of awful books I actually finished reading was Battlefield Earth by Hubbard. A (thin) novella's worth of story spread over a forest worth of pages. A master of the English language can say in two or three words what an average writer says in ten. Hubbard took a hundred.