Dear Booksharians: I have a question about ISBN numbers, recently I've been observing that the books have fronm two to several ISBN's numbers. For example The End A Series of Unfortunate Events book 13 has four: two on trade binding and two on library binding. I know that we can leave that field blank when we submit but I am very curious, for what purpose are they? I mean what is the difference between two trades or two libraries. Dilsia scratching her head wondering