Serial Killer x Stalker! 
NEED I SAY MORE?! Ugh this book honestly had me at the edge of my seat just reading back to back chapters just trying to figure WHAT was going to happen next! It was honestly borderline psychologically romantic the way Cas was toying with Will through anonymous texting (if you consider that a romantic gesture). But you know what? I was sooo down for it. But in all seriousness, if you're looking for a book to make you gasp out loud, look no further.