Ok. Here's the thing. The almost-ending twist seems like it's supposed to be a major revelation. (Don't worry, I won't spoil the twist here.) But at some point in the first third of the book, I subconsciously registered the twist content anyway - so when Jim reveals it to Eileen, my reaction was "Wait, didn't we know that already?" 
 Was there some clue that I caught without realizing it, or did you also feel that the revelation wasn't a surprise?