Do you read Prefaces? Forewards? Introductions? If yes or no, why or why not?
I used to always read them, until I read a Preface that gave away a key plot point rather nonchalantly -- something to the effect of: "When the protagonist's husband dies halfway through the book..." WTF, people? I cannot remember which book, but I know it was considered a classic, so perhaps they assumed that "everyone" knew the ending. Well, I did not. And I've been a lot more careful about what front matter I read ever since!
I mention it because The Screwtape Letters has two Prefaces, both written by C. S. Lewis. One was included in the original published work and the other was written almost 20 years later. I avoided the latter for the reason stated above, but I went ahead and read the original, which did give me some groundwork and didn't give anything away. When I finish the book, I'll go back and read the other one and see if it had any spoilers. I've never found another book that did, but I'm always super nervous now. Jerks.
Meanwhile, I'm 18 pages in and I can already tell this is going to be a verrry difficult book to sum up. So get ready!