Don't Look Back (libro en Inglés) - Jennifer L. Armentrout

My breath caught and I dropped the letter, scuttling back on the bed. Heart racing, I closed my eyes, but I could still see the words... Samantha Franco has the perfect life. Until, that is, she and her best friend Cassie disappear and only Samantha resurfaces... with no knowledge of what happened. And Cassie stays missing. Gradually, Samantha begins to piece together her memories of that night. If she can do that, she may yet be able to save her friend. And, little by little, something begins to emerge... Then she gets the note. Don't look back. You won't like what you find.