This really didn't work for me. The book description is heavily misleading-- where the title and description make it sound as if the disappearance of Nick's sister Dara on her birthday is the central mystery of the story, in fact that disappearance doesn't happen until nearly the end of the book. The vanishing of 9-year-old Madeline Snow, which is also made to sound central, happens earlier but is mostly background noise for the main characters until, again, very near the end. Most of the book is spent on the daily travails of Nick and Dara and their ongoing drama over a half-remembered car accident several months earlier. There's a bit of direction to the story as pictures of their lives before and after that accident gradually become clearer, but for a lot of the time the story just... meanders, without really seeming to go much of anywhere.
Then there's The Twist, and... hm. Yeah. There's not a lot I can say about it without being spoilery, so I won't, but I will say it felt too pat to me, and a bit too like a popular-media-fied version of how the world works. I was heavily skeptical that events could have worked out as portrayed.
With that said: I wouldn't say it's a "bad" book, per se; it just wasn't the one I was expecting or hoping to read, and it really wasn't my thing. I did enjoy Oliver's writing style (this was the first book of hers that I had read) and will probably pick up more of her work at some point to see if I get along better with it.