The Martha's Vineyard Beach and Book Club by Martha Hall Kelly

Mari has come to Martha's Vineyard under the guise of taking a private painting class with artist Elizabeth Devereaux. Mari's mother Nancy died recently and among her things were Elizabeth's name and some items that suggested Nancy was planning a trip to visit Elizabeth. Mari wonders what connection she has with Elizabeth and Copper Pond Farm, Elizabeth's residence. As Mari confesses the real reason she's come to the island, Elizabeth begins to tell her the story of siblings Tom, Cadence, and Briar, as well as their friend Bess, and their grandmother during World War II. It's a story full of love, secrets, Nazi U-boats, and betrayal.