This book should be placed in the same genre as Girl on the Train. A chicklit novel containing a mystery, not a genre mystery. The difference? You should be able to do more with the former - there is leeway in not being part of a genre. Note the word "should".
I keep picking these up because the world seems to go gaga over them, and I must start being more discerning. Here you will find the usual tricks of the trade (and I do mean tricks): women in peril, handsome but mysterious men, a busted sense of priorities. We can't ever be sure of the ground we're standing on, otherwise the book wouldn't have a mystery at its heart.
The problem is, these books require stellar writing to pull the wool over our eyes. Rarely do they do that. I figured out the switcheroo coming in part 2 halfway through part 1 (to be totally honest, I didn't completely figure it out, but I was damn close). This writing duo is trying to pull off the mystery specifically through wordsmithing. If you don't do that perfectly, you foreshadow everything that's coming at the same time you're trying to obscure it.
Plus, this is just pulp fiction. I don't care how authors try to contemporize this - you can dress it up as if it's not a romance or a thriller, but in the end any book that spends a fair number of its words on what people are wearing, how they look and what they buy is not worth my time unless it includes some showoff writing.