This is a pretty good psychological mystery. To begin with, it takes a while to figure out who the good guy is, and even then you aren't 100% sure you've got it right. But then once you settle on someone and you think you've got it all figured out, you see that maybe you were missing something.
It's hard to tell much about this story without also giving a little away. Here's the best I can do: Olivia might be losing her mind. Robert, her husband, might be a control freak. So when they each kidnap their children, the kids might be safe with that parent or their lives may be on the line. The plot has many twists so even when you think you've got it figured out, you're sure to have missed something.
After reading Sleep Tight, I learned that it is actually the third book in a series featuring Detective Tom Douglas. For this book it didn't matter that I hadn't read the first two, it was great on its own.
Fans of Gillian Flynn will like this. I'm adding the others to my "to-read" shelf.