I hate book introductions. Hate them. And yet I always make the mistake of reading them.
Recently, I read Lionel Shriver's We Need to Talk About Kevin. It was preceded by an introduction written by Kate Mosse (I believe, I'd check but somebody else has put the book away, meaning I'll almost definitely never see it again) which I started to read. I stopped when [SPOILER] she told me that Eva was an unreliable narrator. Now, that might not seem like much of a spoiler, but it ruined my enjoyment of the book in a specific way. I was looking for the 'solution'. If the narrator is unreliable, then you have to look closely at what they're saying and decide what's true and what isn't. Now, since most first-person narrators are unreliable (in fact, all of them are unreliable because they're telling what happened from their perspective, because everything they say is coloured by opinion) usually we do this without even really thinking about it. But the fact that Mosse felt the need to specifically point it out to me meant that Eva was, for an actual reason, more unreliable a narrator than most. Which meant there were things to dig out of the text, twist endings to guess. I had about half a dozen different theories at different points in the text, and it was frustrating. I don't mind when I come up with theories naturally, and think "oh, I bet this is going to happen." What bothers me is when I'm actively searching for the 'solution' because I've been told that a character is unreliable.
Kevin isn't the only book that I've had this problem with. My copy of Wuthering Heights came with an introduction that provides a superb and quite detailed summary of the entire novel. So detailed that there was no need to read the book itself, because I already knew the story. I wouldn't start a novel by saying "Once upon a time there was a prince and a princess. The princess was guarded by a dragon and the prince slayed it, then married the princess. Now, sit back while I'll tell you all that in a lot more words." Sure, you might start at the end and work backwards, you might start in the middle and jump all over the place, but you don't tell your reader every plot point right away. So why outline it all in the introduction? It makes no sense to me.
If I'm honest, I don't really understand why books need introductions at all (well, novels, I can see why a non-fiction book might benefit from one) but giving them introductions that spoil the book before you've even read the first line just seem ludicrous to me. Have you ever read an introduction that enhanced a book?