The Lake House, by Kate Morton

I adore Kate Morton.  Sadie Sparrow is a police detective in big trouble for leaking information about a botched investigation to the papers.  In exile in Cornwall, Sadie finds an old abandoned property and learns about the disappearance of a young boy decades earlier.  To prove herself (perhaps to herself), she begins to investigate the circumstances of the child's disappearance and follows the trail to his sister, a mystery writer named Alice Edevane.  Lots of secrets!!