Here it is:
The author that signed this lovely bookmark is Anastasia Hopcus and her book is titled Shadow Hills which is due to come out on July 13, 2010.
Here is the summary (from goodreads):
Since her sister’s mysterious death, Persephone “Phe” Archer has been plagued by a series of disturbing dreams. Determined to find out what happened to her sister, Phe enrolls at Devenish Prep in Shadow Hills, Massachusetts—the subject of her sister’s final diary entry.
After stepping on campus, Phe immediately realizes that there’s something different about this place—an unexplained epidemic that decimated the town in the 1700s, an ancient and creepy cemetery, and gorgeous boy Zach—and somehow she’s connected to it all.
But the more questions she asks and the deeper she digs, the more entangled Phe becomes in the haunting past of Shadow Hills. Finding what links her to this town…might cost her her life.
Thanks again Adrienne!