I loved Gillian Flynn's earlier novel, Dark Places, (except for the ending which I found goofy) and now she has written one of the all time greatest books of the genre!
Gone Girl makes The War of the Roses (movie) look like a nursery rhyme. I could barely put it down. It's not for everyone. I have friends who intensely dislike it. Yes, it's dark, but it's also a crazy mad delight.
I figured out the first part early on and had an interesting theory for the last part which I am putting as a spoiler at this link.