He Said He Would Be Late by Justine Sullivan

A fast-paced, twisty psychological debut about the complexities of marriage and new motherhood, told through the frenetic lens of a wife seeking the truth about her husband, at all costs, as the validity of the life she once knew unravels page by page.
Liz Bennett knows that she is one of the lucky ones. Wealthy and charming, Arno is a supportive husband to Liz and a doting father to their daughter, Emma. A rising banker at a top firm in the Boston area, he is the picture of perfection, rounding off their idyllic New England life. But when Liz sees a text on Arno's phone with a kissy-face emoji, her anxiety kicks into overdrive and she begins to worry that her luck has run out.
Plagued by persistent skepticism and countless sleepless nights, Liz decides she must uncover the truth about her husband—as any wife would. So she takes a deep breath and dives down the rabbit hole. As Liz peels back layers of deceit and tracks down every lead, a frenzy begins to take...