From the Description:
Susanna and Nicholas have a delightfully open marriage in which they promise to keep no secrets. But after a brief tryst on the train back to New York, Susanna finds herself longing for someone she barely knows. While she confesses part of her affair, she leaves out the details that she quickly obsesses over. Back in the city, she and her husband return to their hedonistic lifestyle full of champagne, new lovers, and games of one-upmanship, but Susanna can’t get the man from the train out of her mind…
You can preview the first couple chapters below: