I am willing to bet the vast majority of people who say they would not burn a book, would happily burn a book if it promoted an ideology they found personally disgusting and knew they could get away with it. Have them tell you they would not burn a book because they believe in the free expression of thought and yadda yadda, then present them with the above scenario in which case the book was wholely offensive to them and they would want to (and would if in said scenario they could do it without consequence) suppress it because they do not want the ideology it represented spreading intellectually (whether or not they believe said book had anything intellectually stimulating would necessarily be subjective in this affair)
My opinion is that it is 'what you are in the dark' that counts, There are plenty of non-fascists in this world who would gleefully engage in the burning of books if they knew it would stop the spread of some ideology or thinking or way of thinking they despised, found repugnant or politically abhorrent. I honestly believe very, very few of you would actually resolve to not burn a book in such a scenario. Most people, especially in the modern era, don't count much for self discipline and often have very poorly defined principles (even to themselves) so I honestly believe that most people who said no to this poll, are not being entirely intellectually honest with themselves and are paying lip service to freedom of expression. Who among you wouldn't, if you could, suppress the spread of an increasingly popular modern Communist Manifesto, or Mein Kampf, if burning the books would aid in ceasing the spread of the ideology's influence?
My guess would be 1 in a hundred, and that's a generous estimate.