Paper Town by John Green

Must confess - this is one of my favorite YA books, mystery or regular fiction.  And if you want a hilarious version of what the book is about go to http://www.youtube.com/watch?v=y5ppwMTErrc and watch John Green tell you.  As a matter of fact, go see all of them. I think there are at least 5.
Margo Roth Spiegelman is the next door neighbor to Quentin "Q" Jacobson.  They have been friends until junior high, but not since she became the most popular girl at their high school.  "Q" and his two buddies aren't.  When she shows up at his window one night and asks him to drive her around while she does preposterous things involving large dead fish and hair remover.  Then she disappears, leaving clues which Quentin follows.  The biggest is paper towns which are towns which don't exist but listed on maps.  Go on the journey, it is sooooh worth it.