How do you know?

In the middle of this book I'm reading, I felt something was missing. The writing is exceptional, the setting is great, and the characters are well layered. Still, something is missing. I wish I knew what it was. I'm going to finish it since I'm more than halfway through it.

So my question is, how do you know when a book is great? How do you know when the author has nailed the story? As an author, I know I love every book I write. Some readers agree while others disagree. At the end of the day, I'm satisfied and proud of my work. On the other hand, when I can say "Wow" at the end of the story, then the book is excellent.