I think that the later Dark Tower books could, collectively, be called King's worst book. King doesn't plot out his books in advance (in fact, he's said anyone who does this isn't a real author), and the flaw of this becomes readily apparent when you're trying to write an epic that spans thousands of pages.
I knew I should have stopped reading that series when Stephen King introduced Stephen King as a major character, but I had committed too much time already to not see how it ended. Even worse than that though was the big reveal that: