When someone discovers you are writing a textbook, one (or both) of two questions will be asked. The first is “Why are you writing a book?” and the second is “How is your book different from what’s out there?” The first question is easy to answer. You are writing a book because you are not entirely satisfied with the available texts. The second is harder to answer. The answer can’t be put in a few sentences so, in order not to bore your audience, you try to say something quick and witty. It usually doesn’t work.
-George Casella