The Witness by Nora Roberts
In The Witness, Elizabeth Fitch is the daughter of a controlling and cold mother who is a famous surgeon. When her mother is away at a medical conference, Elizabeth changes her appearance, makes fake IDs for herself and her friend and they go out to one of the hottest night clubs in the area. They drink too much and meet two Russian men who take them back to their house. However, when they get there two other Russian men come and murder her friend and one of the Russian men that brought her there. Elizabeth escapes and goes into a witness protection program.
Ten years later she has a new identity, Abigail Lowery, and is a home security expert. The little town in the Ozarks where she lives is peaceful and quiet. However, the townspeople notice the reclusive young woman who seldom goes out. When she does go out she always has a large attack dog by her side and carries a Glock 19 under her jacket, even at the grocery store. The sheriff of the small town is fascinated by the mysterious woman, but unlike Abigail he has never come into contact with anything as evil as what is coming their way. He doesn’t even know that he is also a target.