I know what you did : a novel / Cayce Osborne.

Enhanced descriptions from Syndetics:

When a bestselling novel fictionalizes the death of her childhood best friend-and accuses her of the murder-Petal Woznewski must figure out who wrote it and why. Truth and fiction collide in this captivating debut novel by Cayce Osborne.

Petal Woznewski is content with her quiet, introverted life in New York City- she has her junk food, her movies, and her on-again, off-again boyfriend, Gus. That peace is shattered when her name appears on the dedication page of an anonymously written thriller with a cryptic note- "I know what you did, Petal Woznewski. And now everyone else will, too."

As she reads, Petal realizes the story is rooted in a secret she buried thirty years earlier, when she was fourteen. A secret involving the tragic death of her friend, Megan. A secret that only one other person knows-their old friend, Jenny. Armed with a copy of the book and her own suspicions, Petal returns to her hometown of Madison, Wisconsin. There, she discovers more questions than answers. Jenny has disappeared, and Petal's old high school crush, Ben, doesn't know anything about the book-at least not anything he's telling.

As sinister clues pile up, and the thriller's plot detours dangerously from the facts, Petal has no choice but to confront her past and solve the mystery of who wrote it-before her very real life ends as tragically as the novel.