A stranger at the door / Jason Pinter.

Enhanced descriptions from Syndetics:

Rachel Marin is in a good place. After years of struggle, the single mother has found both a stable, loving relationship and a new purpose: putting her investigative skills to work solving crimes for the local PD. But just as the pieces of her life are finally starting to fall into place, her teenaged son's teacher is gruesomely murdered, starting a domino effect that shatters her peaceful existence.

When Rachel discovers an ominous email the teacher sent to her just before his death, she knows she must help bring his killer to justice. But soon a figure from her past reappears, threatening to expose Rachel's darkest secrets if she doesn't tread lightly. And when her son is recruited by a shadowy businessman who may be connected to the murder, Rachel knows this has just gotten very, very personal.

Someone out there is dead set on keeping this grisly cover-up good and buried, which means if Rachel's not careful, it's only a matter of time before her dream life becomes her worst nightmare.