I've always thought Smullyan missed the mark with this story. I think the epistemologist would have been right (or at least, could have been right, in a very meaningless Technically Correct sort of way) if Frank had started out saying "I think the book is red", because as a matter of fact he didn't truly wholeheartedly think the book was red. But what Frank actually said was "The book seems red to me", which is an entirely different statement and which, no matter how I think about it, seems to me like it ought to cover the state of mind "the book looks for all the world like it's red, but I still have some nagging doubts about whether my perception is accurate, such that I'm not sure I actually believe it's red." I don't know what it could possibly mean for a person to say that something "seems" red to them, if it doesn't cover that situation. (Honestly, the first time I read this story I thought Smullyan himself was very cleverly working up to that point, but I ended up disappointed.)
A short story by Raymond M. Smullyan, written in 1982.