The Guardian by Nicholas Sparks

Julie Barenson's young husband left her two unexpected gifts before he died—a Great Dane puppy named Singer and the promise that he would always be watching over her. Now four years have passed. Still living in the small town of Swansboro, North Carolina, twenty-nine-year-old Julie is emotionally ready to make a commitment to someone again. But who? Should it be Richard Franklin, the handsome, sophisticated engineer who treats her like a queen? Or Mike Harris, the down-to-earth nice guy who was her husband's best friend? Choosing one of them should bring her more happiness than she's had in years. Instead, Julie is soon fighting for her life in a nightmare spawned by a chilling deception and jealousy so poisonous that it has become a murderous desire...