N is for Night Watcher by Chris Longmuir

Blurb: (from Goodreads) A mysterious stranger arrives in Dundee, Scotland, with a mission to find a new Chosen One to punish. His inner voices guide him to Nicole, a ruthless business woman with a weakness for the husbands of other women.
One of Nicole’s paramours is found hanged and everyone assumes he has committed suicide. However, his estranged wife, Julie, knows better and blames his death on Nicole. Obsessed with the need to punish Nicole, Julie stalks her, unaware that there is another stalker, the deranged and dangerous Night Watcher.

Who will exact punishment on Nicole first?
What price will Nicole have to pay for her misdemeanors?
Will Julie’s mind games drive Nicole over the edge?
And what price will Julie have to pay for her obsession?

Only the Night Watcher knows!