Good question. Every book has some surprises, I would say. That's what keeps you turning the pages. Maybe 'surprise' could be defined as anything that engages your mind with new information, whether it's positive or negative, as you point out.
A really BIG surprise is one that engages first your mind, and then your emotions as well. The last time I was REALLY surprised in a BIG way by a book was probably when I read "What is the What?" by David Eggers, a great American novelist. This book is a non-fiction story that is told in a fiction (novel) format.
What really surprised me was reading about the Lost Boys of the Sudan, about their long journeys to escape the horrors of their homeland. Because of the heat during the day, they were forced to march at night. Once in a while, during these night marches, they would be attacked by a lion, a hungry lion who would pick out a random boy as his next meal and drag him into the jungle to eat him. Realizing that this actually happened, and that it was not a made up story, was rather surprising.