The mystery is somewhat waterlogged by the environs, Nantucket, but Miss Langton's audience -- the younger girls she formerly wrote for and the older ones they grew into -- won't mind since there's lots of local folkways and bylaws, Nantucket having kept itself to itself; lots of instruction about the tides and the shellfish and the shells; a romance; a clever use of Melville with whom Miss Langton is on close speaking/citing terms; and finally the death of a young woman who represented local conservation but. . . . A beach plum.