Some plays are more beguiling and moving than you expect them to be, at least upon hearing the basic outline, and that's definitely the case with Bill Cain's How to Write a New Book for the Bible, which just had its world premiere last night at Berkeley Rep. It's a taut and at moments incredibly sad play, which takes as its subject a son caring for his dying mother in her final months. The play is pure autobiography -- the main character's name is Bill, and Cain has named the other characters after his family as well, mother Mary, brother Paul, and his father Pete -- but Cain has suffused the action with enough subtle, simple theatrical magic to keep it from feeling self-indulgent.