I've held off from this part of the tag for some days now (as you can well see) because for the longest time, I couldn't think of an answer. I have read some wonderful books, yes, but nothing that changed my opinion of anything as such. Some time back, it just struck me that a suitable answer for this would be Vladimir Nabokov's Lolita.
Before reading this, I was a little iffy about picking it up in the first place - the very idea of a grown man embarking on an affair with a young barely teenage girl repulsed me. I thought to myself that maybe I was better off not reading this - it was sure to be absolutely disgusting. However, a few years later, I did read it and I liked it - Nabokov's writing is very poetic and the book, although disturbing, is not icky in the sense that you expect a book concerning a paedophilic relationship to be. For all those who can stomach it, Lolita is a wonderful read - something that changed my opinion of it once I'd finished with the last page.