Paper Towns by John Green
When the cool and beautiful Margo Roth Spiegelman disappears with just three weeks of high school remaining, her not-so-cool neighbor and childhood friend, Quentin, finds that she has left him some obscure yet useful clues for finding her. Quentin and his friends set off from Orlando to New York on the road trip of a lifetime, wearing only their graduation gowns.
P.S. Also read Looking for Alaska and An Abundance of Katherines by John Green.