ScreenHead posted this list of the Top 13 Unfilmable Novels a few days ago, so enjoy if you haven't already.
In addition to the books, they also name the director who might could pull it off, for example:
Unfilmable: Catcher in the Rye
If Anyone Can Do It: Wes Anderson and Noah Baumbach
Not to quibble with them, but I think I'd pick David Gordon Green (All the Real Girls) over the Coen Brothers for Dunces. That's just me.