Now and Forever - Danielle Steel

Although Jessica and Ian Clarke have been married seven years, they insist the thrill and excitement haven't dimmed. At Jessica's urging, Ian has quit his advertising job to become a struggling writer, and she supports him with her successful San Francisco boutique.
Ian's financial dependence on Jessica upsets him more than he admits, and in a moment of bored malaise, Ian's first casual indiscretion will create a nightmare that threatens everything Jessica and Ian have carefully built. What he does changes their lives, and them, perhaps forever, as they struggle to pay the price of his foolhardy affair.