The Last Thing He Told Me by Laura Dave

 “Protect her." That's the last thing Hannah hears from her husband before he disappears after the company he works for is accused of defrauding investors. Hannah knows that her husband, Owen, means that Hannah should protect her teenage stepdaughter, Bailey—but from what or whom? Hannah and Owen haven't been married for long, and Bailey still hasn't fully warmed to Hannah, even though Owen and Bailey had been on their own for years. The Last Thing He Told Me is an unputdownable tale of Hannah's quest to discover who her husband really was. Fans of Harlan Coben will be enthralled.