Between Husbands and Friends — Nancy Thayer

Lucy met Kate when they were both young wives and mothers. In the years since, the two women and their husbands and children have become inseparable, spending every August together in a house on Nantucket. Lucy and Kate, each other's most trusted confidantes, have shared their secrets, from the mundane to the shocking--but there's one explosive secret that Lucy has kept to herself.

As August once again approaches, Lucy must confront the truth she has kept hidden all these years. Now, in the midst of an unexpected family crisis, she must make a decision that could shatter all of their lives.