The Guest Cottage by Nancy Thayer

Sensible thirty-six-year-old Sophie Anderson has always known what to do. She knows her role in life: supportive wife of a successful architect and calm, capable mother of two. But on a warm summer night, she wonders what’s missing from her life. Her husband echoes that lonely question, announcing that he’s leaving her for another woman, Sophie realizes she has no idea what’s next. Impulsively renting a guest cottage on Nantucket from her friend Susie Swenson, Sophie rounds up her kids, Jonah and Lacey, and leaves Boston for a quiet family vacation, minus one.