Any woman/girl who acts in a way that makes her difficult to figure out or understand.
I really like this chick, but she's like a closed book to me.
a guy/girl who doesn't let anyone read them or cant let anyone read them. possibly thought their ex was 'the one' and went over a bad breakup and left thinking they were the best they ever had.
Mike: Dude I dig her but she's a closed book.
Joe: Forget about it man her loss, there's other fish in the sea.