The House Across the Lake by Todd Ritter

Casey Fletcher, a recently widowed actress trying to escape a streak of bad press, has retreated to the peace and quiet of her family’s lake house in Vermont. Armed with a pair of binoculars and several bottles of bourbon, she passes the time watching Tom and Katherine Royce, the glamorous couple living in the house across the lake. They make for good viewing—a tech innovator, Tom is rich, and a former model, Katherine is gorgeous.
 
One day on the lake, Casey saves Katherine from drowning, and the two strike up a budding friendship. But the more they get to know each other—and the longer Casey watches—it becomes clear that Katherine and Tom’s marriage isn’t as perfect and placid as it appears. When Katherine suddenly vanishes, Casey becomes consumed with finding out what happened to her. In the process, she uncovers eerie, darker truths that turn a tale of voyeurism and suspicion into a story of guilt, obsession and how looks can be very deceiving.

With his trademark blend of sharp characters, psychological suspense and gasp-worthy twists, Riley Sager’s The House Across the Lake will shock readers until the very last page.