I Am Princess X by Cherie Priest

Once upon a time, two best friends created a princess together. Libby drew the pictures, May wrote the tales, and their heroine, Princess X, slayed all the dragons and scaled all the mountains their imaginations could conjure. Once upon a few years later, Libby was in the car with her mom, driving across the Ballard Bridge on a rainy night. When the car went over the side, Libby passed away, and Princess X died with her. Once upon a now: May is sixteen and lonely, wandering the streets of Seattle, when she sees a sticker slapped in a corner window. Princess X? When May looks around, she sees the Princess everywhere: Stickers. Patches. Graffiti. There's an entire underground culture, focused around a webcomic at IAmPrincessX.com. The more May explores the webcomic, the more she sees disturbing similarities between Libby's story and Princess X online. And that means that only one person could have started this phenomenon---her best friend, Libby, who lives.