Lying Next to Me by Gregg Olsen

No matter what you see, no matter what you've heard, assume nothing.
Adam and Sophie Warner and their three-year-old daughter are vacationing in Washington State's Hood Canal for Memorial Day weekend. It's the perfect getaway to unplug—and to calm an uneasy marriage. But on Adam's first day out on the water, he sees Sophie abducted by a stranger. A hundred yards from shore, Adam can't save her. And Sophie disappears.
In a nearby cabin is another couple, Kristen and Connor Moss. Unfortunately, beyond what they've heard in the news, they're in the dark when it comes to Sophie's disappearance. For Adam, at least there's comfort in knowing that Mason County detective Lee Husemann is an old friend of his. She'll do everything she can to help. She must.
But as Adam's paranoia about his missing wife escalates, Lee puts together the pieces of a puzzle. The lives of the two couples are converging in unpredictable ways, and the picture is unsettling. Lee suspects that not everyone is telling the truth about what they know—or they have yet to reveal all the lies they've hidden from the strangers they married.