I would have thought that a book with the tagline 'You are happily married. Suddenly your long-lost lover calls. Would you be tempted?' would actually be about...oh, I dunno...being tempted by a long-lost lover. Maybe?
45% into it and aside from a mention in the first chapter of a letter from the aforementioned long-lost lover to his now married ex, I've seen zip about it. It's all about a ton of other dysfunctional people.