Another hard one. I don't know that a book has ever changed my opinion about anything, not in the way I think this question means. Which is probably another sign that I don't read enough and don't look adventurously enough for books outside my comfort zone. I have a lot of catching up to do. But here's the best thing I can think of:
Twelfth Night, by William Shakespeare
This play changed my opinion about... Shakespeare! Haha. That feels like cheating. But it's true. Prior to this play, I'd only read the standard school curriculum fare, and it wasn't doing much for me. I liked Shakespeare sort of because I knew I had to, but I didn't feel a real fondness until I went to college and read this. This play is FUN and not incredibly difficult to follow for a newbie with the language, and it's got cross-dressing and an arguably gay pirate (totally gay pirate). I remember reading it in my dorm and laughing and suddenly realizing that I was laughing at these jokes written in Shakespeare English, and I was so pleased with myself. And suddenly the floodgates were open: I could appreciate just about any of his plays, and the ones I didn't like after that were just... the ones I didn't like! As opposed to didn't get.
So, thanks Twelfth Night! Someday I'll write a movie sequel to you where the apparent happy ending falls apart very quickly, because that is what would happen.