The thing that this book changed for me was the actual reading experience. At the time I first read it my English wasn't what you'd call "good".It was terrible, actually. But I remember I wanted to read it in English, desperately so. I have no idea what made me want to read it in English, but there you have it. I also remember the cover. It looked something like this, only my copy was a shade darker, or so I remember it.
See how he's leaning over her, either to whisper a secret or to take a better look at her, now that she's sleeping? *sigh* I loved the cover! Anyway, I remember I struggled while reading it, but I made it to the end. I remember rereading the book in my mother language years later and feeling a little...cheated? The translation was perfect, flawless, but it still wasn't the same. So you could say that this book changed the way I look at the idea of reading translated books. Of course, if the original language is one that I don't know, I'll read the translation :P
But other than that, I can't say a book changed my opinion about a specific subject.
What about you? Did you ever read a book that changed your opinion about something?