First, this question should be moved to a discussion board. You will receive many different answers there.
I enjoy mystery books because they make me think about what I am reading. As a writer and English teacher, I am drawn to the characterizations of those depicted in this genre. I find the bits and pieces given to the reader over time much more engaging than a text which simply tells a chronological story.
The mystery text is engaging, thought provoking, and (in the really good ones) full of dead ends. It keeps the reader cognitively challenged.
The mystery book also provides for more assumptions to be made over the course of the text. It can build a reader up, or it can break a reader down.