Surprise me / Sophie Kinsella.

After being together for ten years, Sylvie and Dan have all the trimmings of a happy life and marriage; they have a comfortable home, fulfilling jobs, beautiful twin girls, and communicate so seamlessly, they finish each other's sentences. However, a trip to the doctor reveals they could live another 68 years together and panic sets in. They never expected 'until death do us part' to mean seven decades. In the name of marriage survival, they quickly concoct a plan to keep their relationship fresh and exciting: they will create little surprises for each other so that their (extended) years together will never become boring. But in their pursuit to execute Project Surprise Me, mishaps arise and secrets are uncovered that start to threaten the very foundation of their unshakeable bond. When a scandal from the past is revealed that questions some important untold truths, they begin to wonder if they ever really knew each other after all.