The sandwich Catherine Newman serves up in her new novel, “Sandwich,” is a classic one: grown kids on one side, aging parents on the other, and 54-year-old narrator Rachel, a.k.a. “Rocky,” in the middle. As they do every year, three generations of Rocky’s family have decamped to Cape Cod for a week, a gathering made all the more special since son, Jamie, and daughter, Willa, no longer live at home, and their grandparents are becoming quite frail.
