OMG, THIS WAS AWESOME, couldn't put it down. I liked it even better than Gone Girl. The mystery was really intriguing, and I really loved how the perspective shifted between the three different characters, and two places in time. It was so so cool reading how "that day" went down while at the same time following how Libby was just picking up all the clues. 
 (PS - I read this on a beach vacation, mostly while sitting outside in the sunshine. I would definitely recommending reading it that way, vs. at home, alone, in the dark...)