The Husband's Secret by Liane Moriarty

Liane Moriarty's The Husband's Secret tells the story of Cecelia, who has it all--she has three beautiful girls, a doting husband, a successful Tupperware business, is involved with the PTA, and a pillar of the community. Imagine her surprise when she stumbles across a letter from her husband, with one caveat. The letter can only be opened upon his death. Immediately Cecelia imagines that it must contain a deep dark secret.

Rachel has been mourning her daughter since she was senselessly murdered thirty-five years ago. Janie, Rachel's daughter, was a teenager when she was strangled in a local park and left with rosary beads left in her hands. Since Janie's death, Rachel has worked tirelessly to find the killer. When a new video surfaces from shortly before Janie's murder, Rachel thinks she finally found the man responsible.

Tess is a wife, a mother of one, and an advertising executive. Life is going according to plan until her husband and recently slimmed down cousin announce that they've fallen in love. Shocked and bewildered, Tess packs up her son and decides to temporarily move back to her hometown, where she runs into her ex-boyfriend, soon rekindling their relationship.

Cecelia, Rachel, and Tess barely know one another, but their paths will soon collide when they all feel the repercussions of the letter left by Cecelia's husband.