The most surprising plot twist or ending.
The only moment I’ve found surprising in a book is in Harry Potter and the Half Blood Prince. It was that moment where Harry and Professor Dumbledore get back from the cave and find a dark mark over Hogwarts. I think that was the only time I’ve ever put a book down and walked away in shock. I was reading this on release day too. It was torture not being able to talk to anyone about it.