why does the author say that the book "was written by a pillar of salt?"

The narrator also read about the destruction of Sodom and Gomorrah in the hotel room's Gideon Bible. He calls attention to the moment when Lot's wife looks back and is turned into a pillar of salt. He loves her for that act, because it was such a human thing to do. He likes the frailty of that statement and how it relates to his life.