This is actually a very hard question, there have been so many that have changed my perception in someway, but these are the ones that immediately come to mind.
The Book of Laughter and Forgetting, The Joke and, **The Unbearable Lightness of Being ** all by Milan Kundera. I was preparing to spend a month in the Czech Republic and I had wanted to be somewhat familiar with the country. His characters are so vivid, it is hard not to see bits of oneself in them. What was interesting was how he was perceived in the Czech Republic,at least by some of the people that I had spoken to. They felt very betrayed by him; he had not wanted to return after the revolution -and- he was writing in French (His blurbs now identify him as a Franco-Czech author)
Hidden History by Daniel Boorstin. This was the first book of his that I had read. While his points seem only common sensical when you really think about them, he presented them in a very concise way. For example, widespread literacy is a fairly recent development, so it means that whole sectors of civilization are not completely understood because there is little evidence left of them. Another point, history favours the victors. So, in reading accounts of conflicts,you need to remember that the persepective may be skewed to one side.
The Chronicles of Narnia by C.S. Lewis. This was the first set of books that I can remember buying. I read those books to tatters. Its been sometime since I have read them, but I can still bring to mind passages from the books.