Which is the most thought stimulating book you've read?
"The Power of Positive Thinking"
Catcher in the Rye by JD Salinger probably is my winner for fiction. This is because the narrator tells you explicitly he is a liar. That means the reader can wonder how much of the story is Holden Caulfield recasting the events for his own purposes. The reader can even assume specific events were different ways.
For example, I assume that Holden actually had sex with the prostitute, Sunny, and tried to keep from having to pay, even though Holden insists otherwise. I figure he is changing the events in order to hide an embarrassing incident from his audience.
Plagues and Peoples by William H. McNeill is a piece of nonfiction that changed how I think about history. The book is a history of humanity vis-à-vis disease. It turns around a lot of our assumptions of why many major world events turned out the way they did. Disease has been a major player in history.