In the Beginning
I am the sort who reads prefaces, introductions, and acknowlegements. I think that I do this not out of conviction, but our of a certain dimwitted compulsiveness, because the time spent is rarely rewarded. In some cases, authors use an introduction to say things best left unsaid.
Imagine, for a minute, that Jo Rowling had begun HP and the Philosopher's Stone with a few pages noting that she wasn't describing a real person or a real society and that novels, especially fantasy novels, were fiction, more precisely called lies. It might not have inflicted a fatal wound on Harry, but I can't imagine that it would have helped. Rowling avoided that mistake.
Ursula K. LeGuin, in The Left Hand of Darkness, was less wise.