While I Was Gone by Sue Miller

Jo Becker has every reason to be content. She has three dynamic daughters, a loving marriage, and a rewarding career. But she feels a sense of unease. Then an old housemate reappears, sending Jo back to a distant past when she lived in a communal house in Cambridge, Massachusetts. Drawn deeper into her memories of that fateful summer in 1968, Jo begins to obsess about the person she once was. As she is pulled farther from her present life, her husband, and her world, Jo struggles against becoming enveloped by her past and its dark secret.