January Girl

With Sam trying out normal life at Stanford and his father working an undercover gig across the country, Dean is on his own and getting reckless. While investigating a series of mysterious deaths, he runs into a strange witch called Luna Lovegood, who is searching for what she calls a 'Horcrux'. (Seriously?) Suddenly Dean finds himself dealing with evil wizards, broomsticks, Bat-Bogey Hexes, Dementors, mirrors which order him to wear a pointy hat, and much more - all the while trying not to think too much about Luna's last name and whether it's accurate.