Showing False Memory by Dean Koontz

Martie Rhodes, a happily married, successful video games designer, takes an agoraphobic friend to therapy sessions twice a week. Each trip is a grim ordeal, but the experience has brought the two friends even closer together. Then, one morning, Martie experiences a brief, irrational but disquieting fear of... her shadow. When autophobia - one of the rarest and most intriguing phobias known to psychology - is diagnosed, suddenly, radically her life changes, and her future looks dark. Martie's husband, Dusty, loves her profoundly, and is desperate to understand the cause of her autophobia. But as he comes closer to the terrible truth, Dusty himself starts showing signs of a psychological disorder even more frightening than that afflicting Martie...