Twisted Truth by Melinda Leigh

When Detective Seth Harding responds to shots being fired, he faces a familiar sight: a double homicide. However, he is shocked to find a young boy chained in the basement. The terrified child refuses to speak, but Seth knows he is the key to something sinister.
Reluctantly, Seth calls the only social worker he trusts with the traumatized boy—his wife. Carly is recovering from her own trauma suffered on the job, but she can’t turn away from this case, not from another child in trouble. With torrential rain bearing down, threatening to isolate their little town in a killer flood, Seth and Carly know the best way to keep the boy safe from danger is to take him back to their farm.
But danger is as relentless as the storm. And it’s following them all the way home.