The Ten Year Affair by Erin Somers

When Cora meets Sam at a baby group in their small town, the chemistry between them is instant and undeniable. But both are happily married, and neither is the type to cheat. So as their lives intertwine and the romantic tension between them heightens, Cora turns to her imagination: in her head, she and Sam pursue their feelings; in reality, they resist. Yet as the fantasy affair grows ever more intoxicating, it can't help but cast Cora's everyday life - the mushroom problem in the bathroom, her daughter's new fascination with the afterlife, her husband's obsession with podcasts about the history of rope - in a new light. And when the boundaries grow increasingly blurred, she must decide what truly matters.