Kill Your Darlings by Peter Swanson

After 40 years, Thom Graves has become an incredible disappointment to his wife, Wendy - he drinks too much, flirts with other women, and worst of all, is in danger of revealing their dark secret. But how far is Wendy willing to go to stop him? Starting at the end, and ending at the beginning, this is an ingenious mystery about two people's life together, and the secrets which fatally bind them.