Oh, no, I thought; I’m going to hate this: it’s written by two people; it’s in the form of letters; and World War Two has been done to death. I was wrong on all counts about The Guernsey Literary And Potato Peel Pie Society (Dial, $22), by Mary Ann Shaffer and Annie Barrows.  During the Second World War, the Channel Islands were occupied by the Nazis. Shaffer, who wrote most of the book before she became critically ill, imagines that Juliet Ashton, a sophisticated London editor, received a letter from Dawsey Adams, a Literary Society member who found Ashton’s name written on the flyleaf of a book of Charles Lamb’s essays. After Dawsey invited her to visit Guernsey, Juliet’s life was irrevocably changed by the quiet heroism she discovered among the citizens of that island. It’s a charming homage to the power of literature.