The book: Faithful Place by Tana French (No. 3 in the Dublin Murder Squad series)
The hook: All the books in this series have a way of burrowing into your psyche and lingering with you for days or weeks after you finish them, but this one especially so. Alternating between the past and the present, this story follows a detective who never recovered from being dumped by his first love on the day they were supposed to run away together. Then his entire world is turned upside down when her suitcase is found 22 years later in their meeting spot.
Fear factor: This is one of those psychological mysteries that messes with your head more than anything.