Mr. Murder by Dean Koontz

Martin Stillwater has a vivid imagination. It charms his loving wife, delights his two little daughters, and gives him all the inspiration he needs to write his highly successful mystery novels. But maybe Martin’s imagination is a bit too vivid… One rainy afternoon, a terrifying incident makes him question his grip on reality. A stranger breaks into his house, accusing Martin of stealing his wife, his children—and his life. Claiming to be the real Martin Stillwater, the intruder threatens to take what is rightfully his. The police think he’s a figment of Martin’s imagination. But Martin and his family have no choice but to believe the stranger’s threat. And run for their lives. But wherever they go–wherever they hide–he finds them…