Every book has twists and turns that help feed the tension of the story. What is talked about less, I think, is the set-up of those twists.
I read a book a few months ago that seemed like a contemporary novel, but then in the middle of the book, BAM vampires. Excuse me? Where did that come from? I was confused and angry, because it didn't seem like vampires should be in the story at all. It was about boarding school; there was no hint of vampires at all. The twist was way too big that it wasn't believable. The author didn't include enough set-up or clues for me to realize that there was potential for vampires in the book, so when that twist occurred, it didn't make any sense to me.
It's really important to set-up a world shift or character shift, otherwise it leaves the reader feeling cheated and unfulfilled. (If you've never seen HANCOCK with Will Smith (what up West Philly!) and Charlize Theron, this is exactly what happens. I don't recommend the movie unless you want a good example of what not to do). There needs to be careful planning and clues that something is about to happen that might blow your mind, leading up to the twist. It seems a little counter intuitive, right? A twist is supposed to be a shock, a surprise, but just putting in a twist with no set-up can confuse the reader to the point of a turn-off.
A great example of good set-up and twists is Daughter of Smoke and Bone by Laini Taylor. I highly recommend it, not just for the beautiful writing, but the way she weaves her fantastical elements into the world. The set-up of the magical world is not over the top or heavy-handed (like it is in HP1), but has a very good balance, which is hard to do.
Obvi, twists are important to novels, but I think just as important, is the set-up.